Medical devices and applications of polyhydroxyalkanoate polymers

ABSTRACT

Devices formed of or including biocompatible polyhydroxyalkanoates are provided with controlled degradation rates, preferably less than one year under physiological conditions. Preferred devices include sutures, suture fasteners, meniscus repair devices, rivets, tacks, staples, screws (including interference screws), bone plates and bone plating systems, surgical mesh, repair patches, slings, cardiovascular patches, orthopedic pins (including bone filling augmentation material), adhesion barriers, stents, guided tissue repair/regeneration devices, articular cartilage repair devices, nerve guides, tendon repair devices, atrial septal defect repair devices, pericardial patches, bulking and filling agents, vein valves, bone marrow scaffolds, meniscus regeneration devices, ligament and tendon grafts, ocular cell implants, spinal fusion cages, skin substitutes, dural substitutes, bone graft substitutes, bone dowels, wound dressings, and hemostats. The polyhydroxyalkanoates can contain additives, be formed of mixtures of monomers or include pendant groups or modifications in their backbones, or can be chemically modified, all to alter the degradation rates. The polyhydroxyalkanoate compositions also provide favorable mechanical properties, biocompatibility, and degradation times within desirable time frames under physiological conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of allowed prior application U.S.Ser. No. 10/136,449 filed May 1, 2002, now U.S. Pat. No. 6,867,247 whichis a divisional of U.S. Ser. No. 09/535,146 filed Mar. 24, 2000 thatissued as U.S. Pat. No. 6,548,569, which claims priority to U.S. Ser.No. 60/142,238, filed Jul. 2, 1999, and U.S. Ser. No. 60/126,180, filedMar. 25, 1999.

FIELD OF THE INVENTION

The present invention generally relates to polyhydroxyalkanoate (“PHA”)biopolymers and medical uses and application of these materials.

BACKGROUND OF THE INVENTION

In the medical area, a number of degradable polymers have been developedthat break down in vivo into their respective monomers within weeks or afew months. Despite the availability of these synthetic degradablepolymers, there is still a need to develop degradable polymers which canfurther extend the range of available properties, particularlymechanical properties.

Polyhydroxyalkanoates are natural, thermoplastic polyesters and can beprocessed by traditional polymer techniques for use in an enormousvariety of applications, including consumer packaging, disposable diaperlinings and garbage bags, food and medical products. Initial effortsfocused on molding applications, in particular for consumer packagingitems such as bottles, cosmetic containers, pens, and golf tees. U.S.Pat. Nos. 4,826,493 and 4,880,592 describe the manufacture ofpoly-(R)-3-hydroxybutyrate (“PHB”) andpoly-(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate (“PHBV”) films andtheir use as diaper backsheet. U.S. Pat. No. 5,292,860 describes themanufacture of the PHA copolymerpoly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and the use of thesepolymers for making diaper backsheet film and other disposable items.Diaper back sheet materials and other materials for manufacturingbiodegradable or compostable personal hygiene articles from PHBcopolymers other than PHBV are described in PCT WO 95/20614, WO95/20621, WO 95/23250, WO 95/20615, WO 95/33874, WO 96/08535, and U.S.Pat. Nos. 5,502,116; 5,536,564; and 5,489,470.

One of the most useful properties of PHAs which readily distinguishesthem from petrochemically-derived polymers is their biodegradability.Produced naturally by soil bacteria, PHAs are degraded upon subsequentexposure to these same bacteria in either soil, compost, or marinesediment. Biodegradation of PHAs is dependent upon a number of factors,such as the microbial activity of the environment and the surface areaof the item. Temperature, pH, molecular weight, and crystallinity alsoare important factors. Biodegradation starts when microorganisms begingrowing on the surface of the plastic and secrete enzymes which breakdown the polymer into hydroxy acid monomeric units, which are then takenup by the microorganisms and used as carbon sources for growth. Inaerobic environments, the polymers are degraded to carbon dioxide andwater, while in anaerobic environments the degradation products arecarbon dioxide and methane (Williams & Peoples, CHEMTECH, 26:38–44(1996)). While the mechanism for degradation of PHAs in the environmentis widely considered to be via enzymatic attack and can be relativelyrapid, the mechanism of degradation in vivo is generally understood toinvolve simple hydrolytic attack on the polymers' ester linkages, whichmay or may not be protein mediated. Unlike polymers comprising2-hydroxyacids such as polyglycolic acid and polylactic acid,polyhydroxyalkanoates normally are comprised of 3-hydroxyacids and, incertain cases, 4-, 5-, and 6-hydroxyacids. Ester linkages derived fromthese hydroxyacids are generally less susceptible to hydrolysis thanester linkages derived from 2-hydroxyacids.

Researchers have developed processes for the production of a greatvariety of PHAs, and around 100 different monomers have beenincorporated into polymers under controlled fermentation conditions(Steinbüchel & Valentin, FEMS Microbiol. Lett., 128:219–28 (1995)).There are currently only two commercially available PHA compositions:PHB and PHBV. Because of their great compositional diversity, PHAs witha range of physical properties can be produced (Williams & Peoples,CHEMTECH, 26:38–44 (1996)). The commercially available PHAs, PHB andPHBV, represent only a small component of the property sets available inthe PHAs. For example, the extension to break of PHB and PHBV range fromaround 4 to 42%, whereas the same property for poly-4-hydroxybutyrate(“P4HB”) is about 1000% (Saito & Doi, Int. J. Biol. Macromol. 16: 99–104(1994)). Similarly, the values of Young's modulus and tensile strengthfor PHB and PHBV are 3.5 to 0.5 GPa and 40 to 16 MPa, respectively (forincreasing HV content to 25 mol %), compared to 149 MPa and 104 MPa,respectively for P4HB (Saito & Doi, Int. J. Biol. Macromol. 16: 99–104(1994)).

PHB and PHBV have been extensively studied for use in biomedicalapplications, in addition to their commercial use as a biodegradablereplacement for synthetic commodity resins. These studies range frompotential uses in controlled release (see, e.g., Koosha, et al., Crit.Rev. Ther. Drug Carrier Syst. 6:117–30 (1989) and Pouton & Akhtar, Adv.Drug Delivery Rev., 18:133–62 (1996)), to use in formulation of tablets,surgical sutures, wound dressings, lubricating powders, blood vessels,tissue scaffolds, surgical implants to join tubular body parts, bonefracture fixation plates, and other orthopedic uses, as described in PCTWO 98/51812. Wound dressings made from PHB are disclosed in GB 2166354 Ato Webb, et al. One advanced medical development is the use of PHB andPHBV for preparing a porous, bioresorbable flexible sheet for tissueseparation and stimulation of tissue regeneration in injured soft tissuedescribed in EP 754467 A1 to Bowald et al. and EP 349505 A2. Reportshave also described the use of PHBV to sustain cell growth (Rivard, etal., J. Appl. Biomat., 6:65–68 (1995)).

Besides biocompatibility, it is often desired that an implanted medicaldevice should degrade after its primary function has been met. PHB andPHBV, the only PHAs tested as medical implants to date, have shown verylong in vivo degradation periods, of greater than one year for PHB(Duvernoy, et al. Thorac. Cardiovasc. Surgeon 43:271–74 (1995); Malm, etal., J. Thorac. Cardiovasc. Surg. 104:600–07 (1992)). For manyapplications, this very long degradation time is undesirable as thepersistence of polymer at a wound healing site may lead to a chronicinflammatory response in the patient. Slowly degrading PHB patches usedto regenerate arterial tissue have been found to elicit a long term(greater than two years) macrophage response (Malm, et al., Eur. Surg.Res. 26:298–308 (1994)). Macrophages were identified as being involvedin the degradation of the PHB implants and this long term macrophageresponse appears to indicate the presence of persistent, slowlydegrading particulate material originating from the implant. Although aPHB patch used for repair of the pericardium was not seen by ordinarylight microscopy after 12 months implantation, small residualparticulate material was observed by polarized light microscopy (Malm,et al., Scand. J Thor. Cardiovasc. Surg. 26:9–14 (1992)). It is unclearwhether this particulate material remains localized at the implant siteor migrates throughout the body, possibly causing unforeseencomplications. The biological fate, or medical impact of thisparticulate material, cannot be predicted without long term study. Inorder to minimize potential problems associated with slowly degradingPHAs, it is advantageous to utilize resorbable materials with faster invivo degradation rates.

There has been only one report describing the biocompatibility or invivo degradation of any other PHA polymer in biomedical applications(PCT WO 98/51812). U.S. Pat. No. 5,334,698 to Witholt et al. disclosesmedical articles manufactured with an optically active polyesterisolated from Pseudomonas oleovorans cells; however, no examples ordiscussion of fabrication or biocompatibility testing are shown, and nomethods are provided to obtain the polymer in a suitably pure form forin vivo medical use. Since bacteria suitable for production of thesepolymers may also produce an endotoxin, as well as other inflammatorymediators, it is important that the polymer be processed to remove thesecontaminants.

For many applications, the rate of PHA biodegradation is well suited tothe required product lifetime. However, in certain cases it would bedesirable to be able to exert more control over the rate at which thepolymers breakdown in the environment. Such control would extend therange of applications for this class of polymers. For example, a PHAfilm may have suitable mechanical properties to be used as a mulch film,yet not have the most optimum rate of degradation for the application.The ability to be able to control the rate of degradation of the polymerin the environment would thus be a distinct advantage.

U.S. Pat. No. 5,935,506 discloses a PHB stent. The stent construct,which is reported to bioresorb rapidly, contains a large amount ofplasticizer. However, the plasticized PHB approach fails to work, asgreater than 90% stent stenosis was shown at four weeks (see Behrend,American J. Cardiol. p. 45, TCT Abstracts (October 1998); Unverdorben,et al., American J. Cardiol. p. 46, TCT Abstracts (October 1998)). Itwould be advantageous to provide a bioresorbable stent with improvedmechanical properties and no plasticizer.

Thus while the polyhydroxyalkanoates offer a wide range of mechanicalproperties which are potentially useful in medical applications, theiruse particularly in vivo as resorbable polymers has been limited bytheir slow hydrolysis. It would thus be desirable to develop methods forcontrolling the rates of degradation of polyhydroxyalkanoates.

PCT WO 98/51812 discloses methods for making a wide range ofbiodegradable biocompatible polyester materials known aspolyhydroxyalkanoates. These materials are made in high purity, and aresuitable for use in in vivo medical applications.

It is therefore an object of this invention to provide new devices anduses for compositions comprising or derived from polyhydroxyalkanoateswhich degrade more readily in the environment and/or in vivo.

It is another object of this invention to provide methods forfabricating the articles and devices from these compositions.

SUMMARY OF THE INVENTION

Biocompatible polyhydroxyalkanoate compositions with controlleddegradation rates have been developed. The compositions preferablyinclude a biocompatible polyhydroxyalkanoate that has a controlleddegradation rate of less than two years, more preferably less than oneyear, under physiological conditions. The degradation rates of thepolymers can be manipulated through addition of various components tothe polymeric composition, as well as selection of the chemicalcomposition, molecular weight, processing conditions, and form of thefinal polymeric product. The chemical composition can be altered throughselection of monomers which are incorporated into the polymer, byalteration of the linkages, chemical backbone or pendant groups, and/orby manipulation of the molecular weight. The polyhydroxyalkanoatecomposition can contain additives to alter the degradation rates.Porosity can be increased, hydrophilic substances included, and/orsurface area exposed to water increased, all of which will increase therate of degradation. Hydrophobic coatings or incorporation into orblending with hydrophobic substances with the polymers will decrease therate of degradation.

Preferred devices include sutures, suture fasteners, meniscus repairdevices, rivets, tacks, staples, screws (including interference screws),bone plates and bone plating systems, surgical mesh, repair patches,slings, cardiovascular patches, orthopedic pins (including bone fillingaugmentation material), heart valves and vascular grafts, adhesionbarriers, stents, guided tissue repair/regeneration devices, articularcartilage repair devices, nerve guides, tendon repair devices, atrialseptal defect repair devices, pericardial patches, bulking and fillingagents, vein valves, bone marrow scaffolds, meniscus regenerationdevices, ligament and tendon grafts, ocular cell implants, spinal fusioncages, skin substitutes, dural substitutes, bone graft substitutes, bonedowels, wound dressings, and hemostats. The polyhydroxyalkanoatecomposition can be used in both new and existing medical applications,including drug delivery and controlled release of drugs and otherbioactive materials. These polyhydroxyalkanoate compositions can also beused to make or form coatings on a wide variety of devices, includingstents, catheters, and sensors. Their advantages in new and existingapplications can be the use of a biodegradable substitute material inthe application, or the addition of some other desirable characteristicor attribute associated with the application or use, such as amechanical or surface property, physical or chemical property,sterilization technique, biocompatibility, degradation mechanism,packaging preference, and/or a stability issue.

As demonstrated by the examples, these polyhydroxyalkanoatecompositions, such as poly(4HB), have extremely favorable mechanicalproperties, as well as are biocompatible and degrade within desirabletime frames under physiological conditions. These polyhydroxyalkanoatematerials provide a wider range of polyhydroxyalkanoate degradationrates than are currently available.

Methods for processing these materials, particularly for therapeutic,prophylactic or diagnostic applications, or into devices which can beimplanted or injected, are also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of PHA biopolymers broadly divided into groupsaccording to the length of their pendant groups and their respectivebiosynthetic pathways.

FIG. 2 a is a schematic of the pathways by which short pendant groupPHAs are derived. FIG. 2 b is a schematic of the pathways by which longpendant group PHAs are derived.

FIG. 3 is a graph of P4HB degradation in vivo over time (weeks).

DETAILED DESCRIPTION OF THE INVENTION

Medical devices comprising biocompatible polyhydroxyalkanoatecomposition with controlled degradation rates have been developed.

I. Definitions

A “bioerodible polymer” is a water-insoluble polymer that is convertedunder physiological conditions into water soluble materials withoutregard to the specific mechanism involved in the erosion process.“Bioerosion” includes both physical processes (such as dissolution) andchemical processes (such as backbone cleavage). The prefix “bio”indicates that the erosion occurs under physiological conditions, asopposed to other erosion processes, caused for example, by hightemperature, strong acids or bases, UV light or weather conditions. Theterms “bioresorption” and “bioabsorption” are used interchangeably andoften imply that the polymer or its degradation products are removed bycellular activity (e.g., phagocytosis) in a biological environment.

As used herein in reference to polymers, the term “degrade” refer tocleavage of the polymer chain, such that the molecular weight staysapproximately constant at the oligomer level and particles of polymerremain following degradation. The term “completely degrade” refers tocleavage of the polymer at the molecular level such that there isessentially complete mass loss. The term “degrade” as used hereinincludes “completely degrade” unless otherwise indicated.

In the preferred embodiment described herein, the polymer erodes underphysiological conditions in less than two years, more preferably in lessthan one year.

Biocompatible refers to materials that do not elicit a toxic or severeimmunological response following implantation or ingestion.

II. The Polyhydroxyalkanoate (“PHA”) Compositions

(1) Polymer Compositions

As used herein, “PHA materials” contain one or more units, for examplebetween 10 and 100,000, and preferably between 100 and 30,000 units ofthe following formula I:—OCR¹R²(CR³R⁴)_(n)CO—;

-   -   wherein n is an integer, for example between 1 and 15, and in a        preferred embodiment, between 1 and 4; and    -   wherein R¹, R², R³, and R⁴ independently can be hydrocarbon        radicals including long chain hydrocarbon radicals; halo- and        hydroxy-substituted radicals; hydroxy radicals; halogen        radicals; nitrogen-substituted radicals; oxygen-substituted        radicals; and/or hydrogen atoms.

As used herein, the formula —(CR³R⁴)_(n)— is defined as including thefollowing formulas:—CR³R⁴— (where n=1);—CR³R⁴CR^(3′)R^(4′)— (where n=2); and—CR³R⁴CR^(3′)R^(4′)CR^(3″)R^(4″)— (where n=3);

-   -   wherein R³, R⁴, R^(3′), R^(4′), R^(3″), and R^(4″) can be        independently hydrocarbon radicals including long chain        hydrocarbon radicals; halo- and hydroxy-substituted radicals;        hydroxy radicals; halogen radicals; nitrogen-substituted        radicals; oxygen-substituted radicals; and/or hydrogen atoms.        Thus, formula I includes units derived from 3-hydroxyacids        (n=1), 4-hydroxyacids (n=2), and 5-hydroxyacids (n=3).

These units may be the same in a homopolymer, or be more differentunits, as for example in a copolymer or terpolymer. The polymerstypically have a molecular weight over 300, for example between 300 and107, and in a preferred embodiment 10,000 to 10,000,000 Daltons.

The PHA materials may contain or be modified to include other molecules,such as bioactive and detectable compounds, surface active agents, otherdegradable or non-degradable polymers, as well as materials used tomodify the mechanical properties of PHAs such as plasticizers, fillers,nucleating agents, colorants, stabilizers, modifiers and binders.

Representative PHAs which can be modified or formulated as describedherein are described in Steinbüchel & Valentin, FEMS Microbiol. Lett.,128:219–28 (1995).

PHB and P4HB possess very different physical properties. A range of PHAcopolymers containing 4-hydroxybutyrate are either known or can beprepared with a range of intermediate properties between those of PHBand P4HB (Saito & Doi, Int. J. Biol. Macromol. 16:99–104 (1994)).However, biomedical applications, biocompatibility testing, and in vivodegradation of P4HB and its copolymers have not been reported. PHAcopolymers of 4HB and 3HB varying in composition from 0 to 100% 4HB havebeen produced in Alcaligenes eutrophus (Nakamura, et al. Macromol.25:4237–31(1992)) and from 64 to 100% 4HB in Comamonas acidovorans(Saito & Doi, Int. J. Biol. Macromol. 16:99–104 (1994)). However, thesepolymers were of modest molecular mass (1×10⁵ to 5×10⁵ g/mol, by GPC)compared to the molecular mass produced in recombinant E. coli (greaterthan 5×10⁵ g/mol, GPC).

The PHA biopolymers may be broadly divided into three groups accordingto the length of their pendant groups and their respective biosyntheticpathways (FIG. 1). Those with short pendant groups, such aspolyhydroxybutyrate (PHB), a homopolymer of R-3-hydroxybutyric acid(R-3HB) units, are highly crystalline thermoplastic materials, and havebeen known the longest (Lemoigne & Roukhelman, Annales desfermentations, 5:527–36 (1925)). A second group of PHAs containing theshort R-3HB units randomly polymerized with much longer pendant grouphydroxy acid units were first reported in the early seventies (Wallen &Rohwedder, Environ. Sci Technol., 8:576–79 (1974)). A number ofmicroorganisms which specifically produce copolymers of R-3HB with theselonger pendant group hydroxy acid units are also known and belong tothis second group (Steinbuchel & Wiese, Appl. Microbiol. Biotechnol.,37:691–97 (1992)). In the early eighties, a research group in TheNetherlands identified a third group of PHAs, which containedpredominantly longer pendant group hydroxy acids (De Smet, et al., JBacteriol, 154:870–78 (1983)).

The PHA polymers may constitute up to 90% of the dry cell weight ofbacteria, and are found as discrete granules inside the bacterial cells.These PHA granules accumulate in response to nutrient limitation andserve as carbon and energy reserve materials. Distinct pathways are usedby microorganisms to produce each group of these polymers. One of thesepathways leading to the short pendant group polyhydroxyalkanoates(SPGPHAs) involves three enzymes, namely thiolase, reductase and PHBsynthase (sometimes called polymerase). Using this pathway, thehomopolymer PHB is synthesized by condensation of two molecules ofacetyl-Coenzyme A to give acetoacetyl-Coenzyme A, followed by reductionof this intermediate to R-3-hydroxybutyryl-Coenzyme A, and subsequentpolymerization (FIG. 2 a). The last enzyme in this pathway, thesynthase, has a substrate specificity that can accommodate C₃–C₅monomeric units including R-4-hydroxy acid and R-5-hydroxy acid units.This biosynthetic pathway is found, for example, in the bacteriaZoogloea ramigera and Alcaligenes eutrophus. The biosynthetic pathwaywhich is used to make the third group of PHAs, the long pendant grouppolyhydroxyalkanoates (LPGPHAs) is still partly unknown; however, it iscurrently thought that the monomeric hydroxyacyl units leading to theLPGPHAs are derived by the b-oxidation of fatty acids and the fatty acidpathway (FIG. 2 b). The R-3-hydroxyacyl-Coenzyme substrates resultingfrom these routes are then polymerized by PHA synthases (sometimescalled polymerases) that have substrate specificities favoring thelarger monomeric units in the C₆–C₁₄ range. Long pendant group PHAs areproduced, for example, by Pseudomonads.

Presumably, the second group of PHAs containing both short R-3HB unitsand longer pendant group monomers utilize both the pathways shown inFIGS. 2 a and 2 b to provide the hydroxy acid monomers. The latter arethen polymerized by PHA synthases able to accept these units.

In all, about 100 different types of hydroxy acids have beenincorporated into PHAs by fermentation methods (Steinbüchel & Valentin,FEMS Microbiol., Lett., 128:219–28 (1995)). Notably, these include PHAscontaining functionalized pendant groups such as esters, double bonds,alkoxy, aromatic, halogens and hydroxy groups.

A preferred polyhydroxyalkanoate for medical applications ispoly-4-hydroxybutyrate (P4HB). P4HB is biocompatible, resorbable,processable, strong and ductile. Maintenance of breaking strength isanother very important parameter for suturing and stapling materials,especially resorbable ones. As resorbable materials are degraded invivo, their physical and mechanical properties change as the result ofthis degradation. For instance, a resorbable suture will loose most ofits breaking strength, and as such its ability to fix tissue, morerapidly than the time for its complete resorption. Polyglycolic acid(PGA) sutures, for example, will loose most of their strength withinthree weeks in vivo (Vet. Surg. 21;192:355–61), but not be completelyresorbed before six weeks. This loss of mechanical strength is theresult of molecular mass decrease of the polymer. It is important tonote that a number of parameters will affect resorption rates and suturebreaking strength in vivo, such as type of tissue, mechanical stresses,and the presence of infection.

The examples demonstrate that the degradation rate of P4HB in vivo isfast relative to other PHAs, however, its resorption rate is slower thanmany of the materials used as resorbable sutures. Additionally, as shownin Table 7, P4HB implants maintain their molecular mass during theprocess of resorption. This maintenance of molecular mass is expected tobe a benefit for the maintenance of mechanical properties, and as suchbreaking strength of PHAs used as wound closing materials. Because oftheir excellent mechanical properties, maintenance of high molecularmass, processability, biocompatibility and resorbability, P4HB andP4HB-co-HA are useful in a variety of medical devices, including, forexample, resorbable wound closure materials such as suturing andstapling materials, particularly as modified herein to increase theirdegradation rates.

(2) Sources of PHAs

PHA materials which can be modified to alter their degradation rates canbe derived from either a biological source, an enzymatic source, or achemical source. The biological source can be a microorganism or higherorganism such as a plant, and can be derived by genetic engineering.

During the mid-1980's, several research groups were actively identifyingand isolating the genes and gene products responsible for PHA synthesis.These efforts lead to the development of transgenic systems forproduction of PHAs in both microorganism and plants, as well asenzymatic methods for PHA synthesis. Such routes could increase furtherthe available PHA types. These advances have been reviewed in Williams &Peoples, CHEMTECH, 26:38–44 (1996) and Williams & Peoples, Chem. Br.33:29–32 (1997).

Methods which can be used for producing PHA polymers suitable forsubsequent modification to alter their rates of degradation aredescribed, for example, in U.S. Pat. No. 4,910,145 to Holmes, et al.;Byrom, “Miscellaneous Biomaterials” in Biomaterials (Byrom, Ed.); pp.333–59 (MacMillan Publishers, London 1991); Hocking & Marchessault,“Biopolyesters” in Chemistry and Technology of Biodegradable Polymers(Griffin, Ed.), pp. 48–96 (Chapman and Hall, London 1994); Holmes,“Biologically Produced (R)-3-hydroxyalkanoate Polymers and Copolymers,”in Developments in Crystalline Polymers (Bassett Ed.), vol. 2, pp. 1–65(Elsevier, London 1988); Lafferty et al., “Microbial Production ofPoly-b-hydroxybutyric acid” in Biotechnology (Rehm & Reed, Eds.) vol.66, pp. 135–76 (Verlagsgesellschaft, Weinheim 1988); Müller & Seebach,Angew. Chem. Int. Ed. Engl. 32:477–502 (1993); Steinbüchel,“Polyhydroxyalkanoic Acids” in Biomaterials (Byrom, Ed.), pp. 123–213(MacMillan Publishers, London 1991); Williams & Peoples, CHEMTECH,26:3844, (1996); Steinbüchel & Wiese, Appl. Microbiol. Biotechnol,37:691–697 (1992); U.S. Pat. Nos. 5,245,023; 5,250,430; 5,480,794;5,512,669; and 5,534,432; Agostini, et al., Polym. Sci., Part A-1,9:2775–87 (1971); Gross, et al., Macromolecules, 21:2657–68 (1988);Dubois, et al., Macromolecules, 26:4407–12 (1993); Le Borgne & Spassky,Polymer, 30:2312–19 (1989); Tanahashi & Doi, Macromolecules, 24:5732–33(1991); Hori, et al., Macromolecules, 26:4388–90 (1993); Kemnitzer, etal., Macromolecules, 26:1221–29 (1993); Hori, et al., Macromolecules,26:5533–34 (1993); Hocking, et al., Polym. Bull., 30:163–70 (1993); Xie,et al., Macromolecules, 30:6997–98 (1997); U.S. Pat. No. 5,563,239 toHubbs; U.S. Pat. Nos. 5,489,470 and 5,520,116 to Noda, et al. The PHAsderived from these methods may be in any form, including a latex orsolid form.

Identification, cloning and expression of the genes involved in thebiosynthesis of PHAs from several microorganisms within recombinantorganisms allow for the production of PHAs within organisms that are notnative PHA producers. A preferred example is E. coli which is a wellrecognized host for production of biopharmaceuticals and PHAs formedical applications. Such recombinant organisms provide researcherswith a greater degree of control of the PHA production process becausethey are free of background enzyme activities for the biosynthesis ofunwanted PHA precursors or degradation of the PHA. Additionally, theproper selection of a recombinant organism may facilitate purificationof, or allow for increased biocompatibility of, the produced PHA.

The minimal requirements for the synthesis of PHA in a recombinantorganism are a source of hydroxyalkanoyl-CoA and an appropriate PHAsynthase (Gemgross & Martin, Proc. Natl. Acad. Sci. 92:6279–83(1995)).Recombinant PHA producers thus require a biosynthetic pathway for ahydroxyalkanoyl-CoA monomer and a suitable PHA synthase. Production of ahomopolymer requires that the organism produce only one suitablesubstrate for the PHA synthase, as production of multiple substratesresults in the formation of a PHA copolymer. Recombinant organismscontaining a transgene encoding a PHA synthase are sufficient forproduction of P4HB.

In the absence of PHA degradation pathways, the molecular mass of thePHA accumulated in recombinant organisms can be very high. PHB producedin recombinant E. coli has been reported to have molecular mass of 4×10⁶g/mol (Sim, et al., Nature Biotech. 15:63–67 (1997)). The molecular massis important for controlling the physical properties of a given PHA,because the increased molecular mass of PHAs produced in recombinantorganisms can lead to improved material properties, such as increasedtensile strength and ultimate elongation (Kusaka, et al., J. M. S. PureAppl. Chem. A35:319–35 (1998)).

The biosynthesis of P3HB-co-4HB containing a low level of 4HB (1.5%) hasbeen described in recombinant E. coli (Valentin, et al., J. Biotech.58:33–38 (1997)). It is noteworthy that the molecular mass of these PHAswere very high (greater than 1×10⁶ g/mol). Additionally, thebiosynthesis of the P3HB-co-4HB and the homopolymer P4HB in recombinantE. coli have been described (Hein, et al., FEMS Microbiol. Lett.,153:411–18 (1997)).

In addition to using biological routes for PHA synthesis, PHA polymersmay also be derived by chemical synthesis. One widely used approachinvolves the ring-opening polymerization of β-lactone monomers usingvarious catalysts or initiators such as aluminoxanes, distannoxanes, oralkoxy-zinc and alkoxy-aluminum compounds (see Agostini, et al., Polym.Sci., Part A-1, 9:2775–87 (1971); Gross, et ad., Macromolecules,21:2657–68 (1988); Dubois, et al., Macromolecules, 26:4407–12 (1993); LeBorgne & Spassky, Polymer, 30:2312–19 (1989); Tanahashi & Doi,Macromolecules, 24:5732–33 (1991); Hori, et al., Macromolecules,26:4388–90 (1993); Kemnitzer, et al., Macromolecules, 26:1221–29 (1993);Hori, et al., Macromolecules, 26:5533–34 (1993); Hocking & Marchessault,Polym. Bull. 30:163–70 (1993). A second approach involves condensationpolymerization of esters and is described in U.S. Pat. No. 5,563,239 toHubbs, et al. Researchers also have developed chemo-enzymatic methods toprepare PHAs. For example, Xie et al., Macromolecules, 30:6997–98 (1997)discloses a ring opening polymerization of beta-butyrolactone bythermophilic lipases to yield PHB.

Biological production of P4HB or P4HB-co-HA has certain advantages overtraditional chemical synthetic methods. The chemical synthesis of highmolecular mass P4HB (greater than 1×10⁵ g/mol) is difficult due to thetendency of the free acid to lactonize to form the relatively unstrainedand kinetically favored five-membered ring. Thus, polycondensation of4-hydroxybutyric acid is difficult to achieve, while the material thatresults from high pressure ring-opening polymerization reactions ofγ-butyrolactone is of very low molecular mass (Korte & Gelt, PolymerLett., 4:685 (1966)) and would have poor mechanical properties. Analternate synthetic strategy for P4HB, the free radical ring-openingpolymerization of 2-methylene dioxolane, results in a copolymercontaining ring opened and unopened units (Bailey, et al. J Polym. Sci.Polym. Chem. 20:3021–30 (1982); Bailey, J. Polym. Preprints 25:210–11(1984)). 4HB has been successfully co-polymerized with 3HB viaring-opening polymerization (Hori, et al., Polymer 36:4703–05 (1996)).However, the molecular weight of the copolymers was modest (less than1×10⁵ g/mol), especially for compositions with more than 80% 4HB (lessthan 2×10⁴ g/mol). Additionally, many of the catalysts used for thechemical synthesis of polyesters contain toxic metals. These toxiccontaminants can be avoided using a biological process to produce PHAs.

(3) PHA Formulations Having Altered Degradation Rates

a. Additives Altering Degradation Rates

The hydrolysis of polyhydroxyalkanoates is accelerated at acidic orbasic pH's and thus the inclusion of acidic or basic additives orexcipients can be used to modulate the rate of degradation of PHAs. Theexcipients can be added as particulates, can be mixed with any otheradditive or agent incorporated or to be incorporated, or can bedissolved within the polymer. Additives which enhance the rate ofdegradation include inorganic acids such as ammonium sulfate andammonium chloride, organic acids such as citric acid, benzoic acids,peptides, ascorbic acid, inorganic bases such as sodium carbonate,potassium carbonate, calcium carbonate, zinc carbonate, and zinchydroxide, and organic bases such as protamine sulfate, spermine,choline, ethanolamine, diethanolamine, and triethanolamine andsurfactants such as TWEEN™ and PLURONIC™. Such additives are preferablyused at concentrations between 0.1 and 30% by weight.

The rate of degradation may also be enhanced by additives which formpores or otherwise increase the surface area in the polymer or increasethe amorphous content of the polymer. Pore forming agents are generallyadded as particulates and include water soluble compounds such asinorganic salts and sugars which are removed by leaching. Suitableparticles include salt crystals, proteins such as gelatin and agarose,starches, polysaccharides such as alginate and other polymers. Thediameters of the particles may suitably be between nanometers to 500microns. They may also be lyophilizable. Pore forming agents can beincluded in an amount of between 0.01% and 90% weight to volume,preferably at a level between one and thirty percent (w/w, polymer), toincrease pore formation. For example, in spray drying or solventevaporation, a pore forming agent such as a volatile salt, for example,ammonium bicarbonate, ammonium acetate, ammonium chloride or ammoniumbenzoate or other lyophilizable salt, is first dissolved in water. Thesolution containing the pore forming agent is then emulsified with thepolymer solution to create droplets of the pore forming agent in thepolymer. This emulsion is then spray dried or taken through a solventevaporation/extraction process. After the polymer is precipitated, thehardened microparticles are frozen and lyophilized to remove the poreforming agents. Plasticizers, such as the citrate esters, and otherpolymers like atactic polyhydroxyalkanoates, may be added to increasethe amorphous character of the polymer.

Hydrophobic coatings or materials which can be incorporated to increasethe degradation rates include hydrophobic compounds such asphospholipids, cholesterol, and other polymers, as well as surfactants.These materials and methods for forming coatings or incorporation intothe materials are described in WO 96/18420 by Bracco Research SA, WO92/18164 by Delta Biotechnology, Ltd., WO 95/03356 by MassachusettsInstitute of Technology, PCT/US97/03007 by Acusphere, U.S. Pat. No.5,271,961 to Mathiowitz, et al., U.S. Pat. No. 5,711,933 to Bichon, etal., and U.S. Pat. No. 5,705,187 to Unger. Specific examples disclosefatty acids and phospholipids as emulsifiers to stabilize the oil phasein the aqueous phase during emulsion/encapsulation process, with theresult that the microspheres are coated with an outer layer of thesurfactant. The use of additives such as fats, waxes, and high molecularweight hydrocarbon are also disclosed to hydrophobize the polymer wallsand to slow water penetration.

b. Modification of PHA Pendant Groups

An alternative method to alter the rate of degradation of PHA polymersinvolves modification of the polyhydroxyalkanoate pendant groups. Thependant groups may be modified in whole or in part. Pendant groups can,for example, be converted to acidic and basic groups, such as carboxylicacids and amines. These types of groups can enhance degradation byaltering local pH values. Alternatively, the pendant groups can beconverted to reactive groups, such as alcohols and amines, which canCleave the polymer backbone either by an intramolecular orintermolecular reaction. In addition to these conversions, the pendantgroups may also be converted to hydrophilic groups to increase uptake ofhydrolytic agents such as water, or they may be converted to groupswhich would increase the amorphous nature of the polymers. Theprocedures required to carry out functional group conversion of thependant groups are well known to those skilled in the art. One suitablemethod that can be used for preparing a PHA incorporating a unit thatalters the degradation rate of the polymer is disclosed in WO 98/39453by Hein, et al. Suitable pendant groups in PHA polymers which will alterthe rate of degradation can also be derived directly by fermentation.

c. Chemical Modification of PHAs

The rate of hydrolysis of a polyhydroxyalkanoate depends upon a numberof factors. One key factor is the chemical nature or reactivity of theester linkages between the monomers. The rate of degradation of the PHAbackbone can thus be altered by incorporating into the polymer backbonechemical linkages which are more susceptible to hydrolysis, or enzymaticattack. Examples of monomers which can be incorporated intopolyhydroxyalkanoate backbones to alter the rate of degradation of thepolymer are 2-hydroxy acids, such as glycolic acid and lactic acid, andother hydroxyacids which modulate the reactivity of the ester linkage,such as 2-hydroxyethoxy acetic acid. Besides incorporating otherhydroxyacids which yield esters which are more susceptible to hydrolyticor enzymatic attack, other types of functionality may be incorporatedinto the polymer backbone. For example, one or more of the esterlinkages can be replaced by groups such as amide, anhydride, carbonate,or carbamate. Examples of monomers which can be incorporated into thepolyhydroxyalkanoate backbone are aminoacids and aminoalcohols.Moreover, multifunctional monomers can be incorporated into thepolyhydroxyalkanoate backbones, for example, triols or tetraols. Thesetypes of monomer units can also be used to increase or maintainmolecular weight of the polymer by interchain crosslinking, or modifycrystallinity of the polymers.

A variety of methods may be used to incorporate susceptible chemicallinkages into the polyhydroxyalkanoate backbones. For example, co-feedsmay be added during fermentation of PHAs which result in theincorporation of desired monomers. Suitable co-feeds includehydroxyalkoxy acetic acids. These types of monomers may also beincorporated during chemical synthesis from hydroxyacid monomers usingcatalysts, and via coenzyme A derivatives using enzymatic catalysts suchas the PHA synthases.

Susceptible chemical linkages may also be incorporated intopolyhydroxyalkanoate backbones after their initial synthesis. Methods toaccomplish this include chemical transformations such as insertionreactions, irradiation, esterification, transesterification (see, e.g.,Otera, et al., Tetrahedron Lett., 27:2383–86 (1986); Otera J. et al.,Org. Chem., 56:5307–11 (1991), Otera, et al., J. Org. Chem.,54:4013–14(1989); and Otera, et al., J. Chem. Soc. Chem. Commun. 1742–43(1991)), ester metathesis reactions (see, e.g., Stanton & Gagne, J. Am.Chem. Soc., 119:5075–76 (1997)), and reactive blending. In the lattercase, chemical reactions can be carried out in the melt with a catalystpresent. For example, esters or polyesters can be melted withpolyhydroxyalkanoates in the presence of suitable catalysts in order tochemically modify the polyhydroxyalkanoate.

d. Processing of PHAs Including Susceptible Linkages

The polyhydroxyalkanoates may be further manipulated using a wide rangeof polymer processing techniques. Preferred methods for processing thesematerials include solvent casting, melt processing, fiberprocessing/spinning/weaving, extrusion, injection and compressionmolding, and lamination.

III. The Devices and Methods of Manufacture Thereof

The polymer compositions are useful for preparing a variety ofbiodegradable and/or bioresorbable medical devices, or coatings thereon.The biodegradable polymers preferably exhibit a relatively slowbiodegradation, for example, having a in vivo half-life of between threeand six months or less. The polymers preferably have a relatively lowmelting point/glass transition temperature, for example, less than 136°C., and/or are soluble in a non-toxic, non-halogenated solvent, for easeof processing.

Representative devices and applications are described below. State ofthe art materials in these devices and applications can be replacedtotally or partially with the biocompatible polyhydroxyalkanoatesdescribed herein to provide the device specifications, such asdegradation rate and mechanical properties.

(1) Suture Fastener Devices

These devices are typically used to reattach tissue to bone. Often theprocedures involve the attachment of tendon, ligament, or other softtissue to bones in the shoulder, knee, elbow, wrist, hand, and ankle. Inone approach, bone anchors are inserted into the bone and then softtissue such as ligament or tendon may be sutured to the anchor point.The procedure may be performed in an open manner or preferably using aminimally invasive technique whereby the device is deployed by asuitable delivery device. Examples of suture fastener devices currentlyin use which are representative of the state of the art include theBionx Biodegradable Anchor (Bionx Implants, Bluebell, Pa.), BioROC EZ™Suture Bone Fastener (Innovasive Devices, Marlborough, Mass.),Resorbable Soft Tissue Attachment Device (Zimmer, Warsaw, Ind.) and theAcufex TAG Bioabsorbable Anchors (Smith & Nephew Endoscopy, Mansfield,Mass.). Polyhydroxyalkanoate suture fastener devices can be fabricatedaccording to the methods and procedures described in U.S. Pat. Nos.5,814,071; 5,797,963; 5,735,875; 5,725,529; 5,649,963; 5,643,321;5,593,425; 5,423,821; 5,269,809; 5,268,001; 5,163,960; and 5,041,129.

(2) Meniscus Repair Devices

A number of devices exist for the repair of meniscus lesions. In oneprocedure, these orthopedic fixation devices are used for the securefixation of longitudinal vertical meniscus lesions (bucket-handlelesions) located in the vascularized area of the meniscus in combinationwith suitable immobilization. The devices are often used in minimallyinvasive surgery. Examples of repair devices currently in use which arerepresentative of the state of the art include the BIOFIX™ MeniscusArrow (Bioscience, Inc., Malvern, Pa.), T-Fix Suture Bar (AcufexMicrosurgical, Inc.), and the Meniscal Dart (Innovasive Devices,Marlborough, Mass.). Polyhydroxyalkanoate meniscus repair devices can befabricated according to the methods and procedures described by de Goot,Biomaterials, 18:613–22 (1997), and in U.S. Pat. Nos. 5,855,619;5,853,746; 5,725,556; 5,645,589; 5,059,206; 5,035,713; 4,976,715;4,924,865; 4,895,148; and 4,884,572.

(3) Rivets and Tacks

Biodegradable rivets and tacks can be used in soft tissue reattachment.Particular uses include the reattachment of soft tissue in the shoulder,including instability repairs in the shoulder (Bankart procedures), SLAPlesion repair, acromio-clavicular separation repairs, rotator cuffrepairs, capsular shift or capsulolobral reconstructions, bicepstenodesis, and deltoid repair. An example of the state of the art rivetdevice is the LactoSorb Pop Rivet (Biomet, Inc., Warsaw, Ind.).Polyhydroxyalkanoate rivet and tack devices can be fabricated accordingto the methods and procedures described by Speer, et al, Clin. Orthop.291:67–74 (1993), and U.S. Pat. Nos. 5,840,078; 4,895,148; 5,868,747;5,843,084; 5,840,078; 5,827,298; 5,807,292; 5,785,713; 5,730,744.

(4) Staples

Biodegradable staples can be used for the fixation of soft tissues. Suchstaples can be used, for example, to repair vertical longitudinal fullthickness tears (i.e. bucket-handle) of the meniscus. An example of suchstate of the art devices include the Absorbable Implantable Staple(United States Surgical Corporation, Norwalk, Conn.).Polyhydroxyalkanoate staples can be fabricated according to the methodsand procedures described by U.S. Pat. Nos. 5,728,116; 5,423,857;5,345,949; 5,327,914; 5,222,963; 4,889,119; 4,741,337; 4,646,741;3,797,499; and 3,636,956.

(5) Screws

Biodegradable screws, including interference screws, can be used in thefixation of soft tissue. Such screws can be used, for example, to fixsoft tissue grafts to bone during cruciate ligament reconstructionsurgeries of the knee. Examples of such state of the art screws includethe RCI screw (Smith & Nephew, Carlsbad, Calif.) and the ArthrexBIO-INTERFERENCE™ Screw (Arthrex, Naples, Fla.). Polyhydroxyalkanoatescrews can be fabricated according to the methods and proceduresdescribed in U.S. Pat. Nos. 5,275,601; 5,584,836; 5,364,400; 5,348,026;5,876,455; 5,632,748; 5,496,326; 5,718,706; 5,690,222; 5,383,878;5,425,733; 5,417,692; 4,927,421; 5,211,647; 5,116,337; and 4,927,421.

(6) Bone Platesand Bone Plating Systems

Biodegradable fixation systems consisting of plates, plates and mesh,and mesh, in varying configurations and length, can be attached to bonefor reconstruction. Such uses include the fixation of bones of thecraniofacial and midfacial skeleton affected by trauma, fixation ofzygomatic fractures, or for reconstruction. The plates may also becontoured by molding. Examples of such state of the art devices includethe Howmedica LEIBINGER™ Resorbable Fixation System (Howmedica,Rutherford, N.J.), and the LACTOSORB™ Trauma Plating System (Biomet,Inc., Warsaw, Ind.). Polyhydroxyalkanoate bone plates and bone platingsystems can be fabricated according to the methods and proceduresdescribed by U.S. Pat. Nos. 5,853,746; 5,735,875; 5,725,529; 5,717,030;5,662,710; 5,626,611; 5,578,046; 5,373,860; 5,092,883; 4,988,358;4,683,878; and 3,997,138.

(7) Surgical Mesh

Biodegradable surgical mesh may be used in general surgery. For example,surgical meshes are used in the treatment of hernias where theconnective tissue has ruptured or as a sling material to support therepositioning and support of the bladder nect for female urinaryincontinence. Such meshes (plugs) may also be used as soft tissueimplants for reinforcement of soft tissue, for example, in the repair ofabdominal aponeuroses and the abdominal wall, fascial and capsulardefects, and patellar and achilles tendons, and replacement ofinfraspinatus thedons and cranial cruciate ligaments. Other uses includethe bridging of fascial defects, as a trachea or other organ patch,organ salvage, slings (including an intestinal sling), dural graftingmaterial, wound or burn dressing, and as a hemostatic tamponade.Examples of such state of the art meshes include the BrennenBiosynthetic Surgical Mesh Matrix (Brennan Medical, St. Paul, Minn.),GORE-TEX™ Patches (Gore, Flagstaff, Ariz.), and SEPRAMESH™ (GenzymeCorporation, MA). Polyhydroxyalkanoate surgical meshes can be fabricatedaccording to the methods and procedures described by Bupta, “Medicaltextile structures: an overview” Medical Plastics and Biomaterials, pp.16–30 (January/February 1998) and by methods described in U.S. Pat. Nos.5,843,084, 5,836,961; 5,817,123; 5,747,390; 5,736,372; 5,679,723;5,634,931; 5,626,611; 5,593,441; 5,578,046; 5,516,565; 5,397,332;5,393,594; 5,368,602; 5,252,701; 4,838,884; 4,655,221; 4,633,873;4,441,496; 4,052,988; 3,875,937; 3,797,499; and 3,739,773.

(8) Repair Patch

Biodegradable repairpatches may be used in general surgery. For example,these patches may be used for pericardial closures, the repair ofabdominal and thoracic wall defects, inguinal, paracolostomy, ventral,paraumbilical, scrotal, femoral, and other hernias, urethral slings,muscle flap reinforcement, to reinforce staple lines and long incisions,reconstruction of pelvic floor, repair of rectal and vaginal prolapse,suture and staple bolsters, urinary and bladder repair, pledgets andslings, and other soft tissue repair, reinforcement, and reconstruction.Examples of such state of the art patches include the TISSUEGUARD™product (Bio-Vascular Inc., St. Paul, Minn.). Polyhydroxyalkanoaterepair patches can be fabricated according to the methods and proceduresdescribed in U.S. Pat. Nos. 5,858,505; 5,795,584; 5,634,931; 5,614,284;5,702,409; 5,690,675; 5,433,996; 5,326,355; 5,147,387; 4,052,988, and3,875,937.

(9) Sling

Biodegradable slings can be used as implants to reinforce soft tissuewhere weakness exists. Examples of such procedures include pubourethralsupport and bladder support, urethral and vaginal prolapse repair,reconstruction of the pelvic floor, and sacro-colposuspension. Thedevice can be used to treat female urinary incontinence resulting fromurethral hypermobility or intrinsic sphincter deficiency. Examples ofsuch state of the art devices include the Mentor SUSPEND™ Sling (MentorCorporation, Minneapolis, Minn.). Polyhydroxyalkanoate sling devices canbe fabricated according to the methods and procedures described in U.S.Pat. Nos. 5,700,479; 5,860,425; 5,836,315; 5,836,314; 5,813,408;5,690,655; 5,611,515; 4,217,890.

(10) Cardiovascular Patch

Biodegradable cardiovascular patches may be used for vascular patchgrafting, (pulmonary artery augmentation), for intracardiac patching,and for patch closure after endarterectomy. Examples of similar state ofthe art (non-degradable) patch materials include Sulzer VascutekFLUOROPASSIC™ patches and fabrics (Sulzer Carbomedics Inc., Austin,Tex.). Polyhydroxyalkanoate cardiovascular patches can be fabricatedaccording to the methods and procedures described in U.S. Pat. Nos.5,716,395; 5,100,422, 5,104,400; and 5,700,287; and by Malm, et al.,Eur. Surg. Res., 26:298–308 (1994).

(11) Sutures

Biodegradable sutures are used generally for soft tissue approximationwhere only short term wound support is required. Examples of similarstate of the art devices include VICRYL RAPIDE™ (Ethicon, Inc.,Somerville, N.J.). Polyhydroxyalkanoate suture devices can be fabricatedaccording to the methods and procedures described in Wound ClosureBiomaterials and Devices, (Chu, et al., Eds.) CRC Press, Boca Raton,Fla., 1996.

(12) Orthopedic Pins

Biodegradable pins, including bone filling augmentation material, areused for bone and soft tissue fixation. Such devices have been used, forexample, to stabilize wrist, foot, ankle, hand, elbow, shoulder and kneefractures. Examples of such state of the art devices include the BIOFIX™Biodegradable Fixation Rod (Davis & Geck, Danbury, Conn.), ORTHOSORB™pins (Johnson & Johnson, New Brunswick, N.J.) and the RESOR-PIN™Resorbable Membrane Pin (Geistlich-Pharma, Washington, D.C.).Polyhydroxyalkanoate orthopedic pins can be fabricated by conventionalprocessing techniques such as melt processing techniques like injectionand compression molding, fiber forming, as well as solvent basedtechniques.

(13) Adhesion Barriers

Biodegradable adhesion barriers are used in general surgery to preventundesirable adhesions, particularly following surgery. Examples of suchstate of the art devices used for these purposes include the EndopathINTERCEED™ Absorbable Adhesion Barrier (Ethicon, Inc.), and SEPRAFILM™(Genzyme, Cambridge, Mass.). Polyhydroxyalkanoate adhesion barriers canbe fabricated according to the methods and procedures described in U.S.Pat. Nos. 5,824,658; 5,795,584; 5,791,352; 5,711,958; 5,639,468;5,626,863; 5,626,622; 5,607,686; 5,580,923; 5,137,875, and 4,840,626.

(14) Stents

Stents are currently used in a range of medical applications, normallyto prevent reocclusion of a vessel. Examples include cardiovascular andgastroenterology stents. Generally these stents are non-degradable.Ureteric and urethral stents are used to relieve obstruction in avariety of benign, malignant and post-traumatic conditions such as thepresence of stones and/or stone fragments, or other ureteralobstructions such as those associated with ureteral stricture, carcinomaof abdominal organs, retroperitoneal fibrosis or ureteral trauma, or inassociation with Extracorporeal Shock Wave Lithotripsy. The stent may beplaced using endoscopic surgical techniques or percutaneously. Examplesof state of the art stents include the double pigtail ureteral stent (C.R. Bard, Inc., Covington, Ga.), SpiraStent (Urosurge, Coralville, Iowa),and the Cook Urological Ureteral and Urethral Stents (Cook Urological,Spencer, Ind.).

One advantage of polyhydroxyalkanoate stents is their bioabsorbability,which is particularly desirable in applications such as urologicalapplications, since a second procedure is not required to remove thestent. Furthermore, one of the main problems in using metallic stents incardiovascular applications is the subsequent restenosis caused byexcessive growth of the endothelial wall, which is believed due, atleast in part, to irritation caused by the metallic stent on the vesselwall (see Behrend, American J. Cardiol. p. 45, TCT Abstracts (October1998); Unverdorben, et al., American J. Cardiol. p. 46, TCT Abstracts(October 1998)). A bioabsorbable stent made from, or coated with apolyhydroxyalkanoate should produce reduced or no irritation.

Polyhydroxyalkanoate stents can be fabricated according to the methodsand procedures described in U.S. Pat. Nos. 5,792,106; 5,769,883;5,766,710; 5,670,161; 5,629,077; 6,551,954; 5,500,013; 5,464,450;5,443,458; 5,306,286; 5,059,211, and 5,085,629. See also Tanquay,Cardiology Clinics, 23:699–713 (1994) and references therein, and Talja,J. Endourology, 11:391–97 (1997).

(15) Guided Tissue Repair/Regeneration

Guided tissue regeneration is a periodontal procedure wherein a membraneis placed over bone and root surfaces of a surgically exposed area. Themembrane acts as a barrier isolating the healing bone and periodontalligament from the gum, giving the slower-growing bone and ligament cellsan opportunity to regenerate. The ultimate goal is to strengthen theattachment of the tooth to the jawbone, thereby providing improvedchances of preserving the tooth or teeth. Examples of state of the artmembranes that are used in the procedure include GUIDOR™ (ProcordiaOratech A. B., Sweden), Gore Resolut XT™ (W. L. Gore & Associates,Flagstaff, Ariz.), VICRYL™ Periodontal Mesh (Ethicon, Sommerville,N.J.), and ATRISORB™ Bioabsorbable GTR barrier (Atrix Laboratories, Ft.Collins, Colo.). Polyhydroxyalkanoate guided tissue repair barriers maybe fabricated according to methods described in U.S. Pat. Nos.5,853,746; 5,736,152; 5,543,441; 5,508,036; 5,455,041; 5,368,859;5,077,049; 5,278,201; 5,250,584; 5,077,049; and 4,938,763.

(16) Articular Cartilage Repair

Biodegradable polymer matrices, alone or incorporating cell and/orbioactive molecular growth factors, have been used to repair articularcartilage defects. An example of a material used in such a procedure ispolylactic acid (Schroder, et al., J Biomed. Mat. Res., 25:329–39(1991)). Polyhydroxyalkanoate articular cartilage repair devices can befabricated according to the methods and procedures described in U.S.Pat. Nos. 5,876,452; 5,736,372; 5,716,981; 5,700,774; 5,655,546; and5,041,138.

(17) Nerve Guides and Tendon Repair

Biodegradable devices may be used as guides to facilitate the regrowthand reconnection of severed or damaged nerves and tendons. The devicesare generally fabricated as tubes. An example of a nerve guide is theNeurotube™ product. Polyhydroxyalkanoate tendon repair devices may beprepared according to procedures described in U.S. Pat. No. 4,792,336.Polyhydroxyalkanoate nerve guides can be fabricated according to themethods and procedures described in U.S. Pat. Nos. 5,800,544; 5,735,863;5,584,885; 5,514,181; 5,026,381; 5,019,087; and 4,955,893.

(18) Atrial Septal Defect Repair

Large atrial septal defects that cannot be closed directly with suturescan be repaired with pericardial patches or with syntheticnon-absorbable materials. Polyhydroxyalkanoate atrial septal defectrepair patches and devices can be fabricated according to the methodsand procedures described in U.S. Pat. Nos. 5,853,422; 5,634,936;5,861,003; 5,855,614; and by Malm, T. et al., Scand. J. Thor.Cardiovasc. Surg., 26:9–14 (1992).

(19) Pericardial Patch

Reoperation after open heart surgery is often made more difficult due toadhesions. Prevention of adhesions through pericardial substitution istherefore becoming more desirable. Many different types of materialshave been used as pericardial patches including silicone membranes,polyurethane, fascia lata, Gore-Tex™, pericardium xenografts, dura materand siliconized Dacron™. Pericardial patches derived frompolyhydroxyalkanoates may be derived according to methods described byGabbay, Ann. Thorac. Surg., 48:803–12 (1989); Heydorn, J. Thorac.Cardiovasc. Surg., 94:291–96 (1987), and U.S. Pat. Nos. 5,713,920;5,468,253; and 5,141,522.

(20) Bulking and Filling Agents

Bulking agents are commonly used in plastic surgery to fill in defects,and also in the treatment of adult incontinence where they are used assphincter bulking materials. An example of such a material is collagen.Polyhydroxyalkanoate bulking and filling devices can be fabricatedaccording to the methods and procedures described in U.S. Pat. Nos.5,376,375; 5,702,715; 5,824,333; 5,728,752; 5,599,852; 5,785,642;5,755,658; and 5,728,752.

(21) Vein Valves

Venous leg ulcers occur on the lower leg and are caused by venousinsufficiency, or poorly functioning valves in the veins of the legs.Currently, there is no treatment available to repair defective veinvalves. Instead, only the ulcers are treated at an estimated averagecost of $2,700 per patient per year (for the estimated 600,000 patientssuffering from venous leg ulcers). It would therefore be desirable toprovide replacement vein valves, which preferably can be implanted by aminimally invasive means, or by routine surgery. Vein valves can bederived from polyhydroxyalkanoate polymers, wherein these polymers arefashioned into a valve structure. The polymers may be used alone,coated, or modified with another agent, such as a biological factor.They may be combined with other materials, and/or made porous.Alternatively, the polymers may be fashioned into scaffolds which canoptionally be cell seeded prior to implantation. Suitable methods toprepare valves and seed tissue engineered scaffolds are described inBreuer et al., Biotechnology and Bioengineering, 50:562–67 (1996);Niklason et al., Science, 284:489–93 (1999); and Principles of TissueEngineering (Lanza, et al., Eds.) Academic Press, Austin, 1997.

(22) Bone Marrow Scaffolds

A number of different surgical procedures employ bone marrowtransplants. In many cases, bone marrow is taken from the iliac crestand used at another location to aid in the repair of tissues and organs,usually bone. Examples include the use of bone marrow in the repair ofbone fractures, such as a tibial plateau fracture, spinal procedures, aswell as treatment of abnormalities in the maxillofacial and craniofacialregions requiring surgery. In certain cases, large amounts of bonemarrow are required for these procedures, but the amount of bone marrowavailable is limited, particularly in young and small patients. Ittherefore is desirable to provide a method which allows the amount ofavailable bone marrow to be utilized effectively over a greater portionof the surgical site, and/or at additional sites, without losing any ofits desirable properties for repair and regeneration. It also may bedesirable to provide the bone marrow in a more useful form forsubsequent surgical use.

The effective coverage or placement of useful bone marrow can beincreased by mixing, seeding, or combining the bone marrow with a porouspolyhydroxyalkanoate polymer. The latter scaffold could be prepared, forexample, by a salt leaching technique described in Principles of TissueEngineering (Lanza, et al., Eds.) Academic Press, Austin, 1997. Afterharvesting, the bone marrow is taken up into the desiredpolyhydroxyalkanoate scaffold by, for example, applying suction orpressure, or by other means. The polyhydroxyalkanoate scaffold also cancomprise other materials, such as bioactive agents, other polymers, orcalcium phosphate, and can be shaped or molded for a particular use. Thescaffold containing the bone marrow may then be applied to the desiredsurgical sites.

(23) Meniscus Regeneration

An unmet need in meniscus repair relates to defects located in theavascular region of the menisci where no blood vessels are present.Tears or defects in this region are not expected to heal. The onlyavailable treatment for avascular tears is a meniscectomy, where theportion of the meniscus surrounding the tear is removed. This procedureis unsatisfactory, as the meniscectomy disturbs the ability of themeniscus to function properly.

Therefore, a meniscus regeneration product able to facilitate repair ofavascular tears is highly desirable. Certain polyhydroxyalkanoates havedesirable properties for use as a meniscus regeneration template. Theseproperties include elasticity, flexibility, appropriate compressivestrength, and controlled bioabsorption rates. The regeneration templatecould also incorporate growth factors and/or cells. Polyhydroxyalkanoatemeniscus regeneration devices can be fabricated using a variety ofdifferent processing techniques, including the use of salt leaching,melt, solvent, fiber, and foam processing techniques. Devices may beformed, for example, as sponges, foams, non-wovens materials, wovenmaterials. Suitable methods for fabricating polyhydroxyalkanoatemeniscus regeneration templates are described by Widmer & Mikos,“Fabrication of biodegradable polymer scaffolds for tissue engineering”in Frontiers in Tissue Engineering (Patrick, et al., Eds.) Ch. 11.5, pp.107–20 (Elsevier Science, New York, 1998).

(24) Ligament and Tendon Grafts

The anterior cruciate ligament (ACL) is a broad, thick cord, about thesize of a person's index finger, which is essential for guiding thetibia (shinbone) in a normal path along the end of the femur (thighbone)and maintaining stability of the knee joint. When this ligament is tornor ruptured, the joint loses stability and further destruction of thearticular and meniscal cartilage results, i.e. degenerative arthritis.The serious injury often results from a sporting accident and usuallyrequires surgical repair or reconstruction. The most commonreconstruction of the ACL involves the use of patellar tendon andhamstring grafts, with cadaver grafts representing a third option.(Suturing is sometimes an option, but 50% of these procedures arereported to fail because of the strain placed on the knee.) The patellargraft is usually harvested with a piece of the patient's patella (kneecap) bone along with a piece of bone from where the patellar tendoninserts into the tibia. It is considered to be a strong donor material,but can increase sensitivity of the patella and tibia where the bone isremoved. The hamstring graft is taken from tendons on the inner side ofthe knee which does not interfere with the patella and tendon; however,this graft is weaker than the patellar graft.

A device known as a ligament augmentation device (LAD) was introducedfor these reconstructive procedures when it was observed that biologicalgrafts undergo a period of degeneration and loss of strength beforebeing incorporated. The LAD is meant to function to protect the graftduring this vulnerable phase, and has been shown to share loads incomposite grafts, increasing the strength of the ligament graft by asmuch as 50%. However, current devices are thought to induce inflammatoryresponses in the knee, so their routine use in uncomplicatedreconstructions has been limited.

Polyhydroxyalkanoate polymers can be used to fabricate bioabsorbableLADs and other ligament and tendon graft devices. The advantages ofthese devices rests in their improved biological response combined withtheir ability to provide early strength to the autograft. Suitabledevices can be fabricated, for example, by processing thepolyhydroxyalkanoates into fibers to be used alone, or after furthermodification into braided or multi-filament forms. Suitable methods forpreparing these devices with polyhydroxyalkanoates are described in U.S.Pat. No. 4,792,336 to Hlavacek, et al.

(25) Bone-Graft Substitute

About 500,000 surgical operations annually require the use of bonegrafts, for example, in spinal fusions, trauma fractures, and inperiodontal surgery. In a typical procedure, bone graft material isharvested surgically from the patient's own hipbone and then insertedinto the grafting site where bone regrowth is desired. The graftmaterial contains a variety of bone promoting agents which helpstimulate the formation of new bone and healing. This procedurefrequently provides good results, but undesirably requires a secondoperation to harvest the autograft. To avoid the harvesting procedure,surgeons may use other types of bone graft substitutes including cadaverbone products and composites containing calcium phosphate and calciumcarbonate. The latter materials generally do not perform well, anddisease transmission issues always accompany the use of cadaver-derivedmaterials.

For these reasons, significant efforts are underway to develop new bonegraft substitutes based on the use of osteoconductive (bone scaffolding)and/or osteoinductive (new bone from biological stimulation) materials.It has become increasingly apparent that these materials require acarrier vehicle for optimum performance. Polyhydroxyalkanoate polymerscan be used as carrier vehicles. Such devices may be fabricatedaccording to procedures described by Widmer & Mikos, “Fabrication ofbiodegradable polymer scaffolds for tissue engineering” in Frontiers inTissue Engineering (Patrick, et al., Eds.) Ch. II.5, pp. 107–20(Elsevier Science, New York, 1998); Damien, et al., J. Appl. Biomater.2(3):187–208 (1991); and Gugala, et al., J. Orthop Trauma, 13(3):187–95(1999).

(26) Skin Substitutes

Skin loss due to either burns or ulcers is a major medical problem. Insevere cases, treatment frequently employs autografts which are takenfrom the patient. However, this source of skin is limited, and theprocedure results in additional morbidity and scarring. A potentialsolution to these problems lies in the development of human skinsubstitutes based upon cell seeded, or tissue engineered, matrices. Thematrices may be derived from bioabsorbable polymers such aspolyhydroxyalkanoate polymers, which can provide a wide range ofproperties and fabrication options needed to produce suitable skinsubstitutes. For example, advantages of polyhydroxyalkanoates in theseproducts include the stability of the polyhydroxyalkanoate matrix tocell culture, improved wound healing due to the use of a lessinflammatory matrix material, and ease of use, such as flexibility andsuturing. Polyhydroxyalkanoate polymers may be fabricated into suitablematrices for use as skin substitutes using procedures described byWidmer & Mikos, “Fabrication of biodegradable polymer scaffolds fortissue engineering” in Frontiers in Tissue Engineering (Patrick, et al.,Eds.) Ch. II.5, pp. 107–20 (Elsevier Science, New York, 1998).

(27) Dural Substitutes

Following neurosurgical operations, cadaveric dura mater grafts havecommonly been used to repair dural defects. However, because of the riskof transmitting Creutzfeldt-Jakob disease through these grafts, theWorld Health Organization has recommended that cadaveric dural grafts nolonger be used. Although polytetrafluoroethylene can be used as analternative permanent synthetic material for dural repair, concernsrelating to the material's biocompatibility have been raised, increasinginterest in the development of a bioabsorbable dural substitute.Polyhydroxyalkanoates with appropriate flexibility and strength can beprocessed into devices suitable for use as dural substitutes. Thesedevices may take the form of porous materials, and can be derived forexample from porous polyhydroxyalkanoate matrices, and/orpolyhydroxyalkanoate fibers processed into webs, non-woven or wovenfabrics. Widmer & Mikos, “Fabrication of biodegradable polymer scaffoldsfor tissue engineering” in Frontiers in Tissue Engineering (Patrick, etal., Eds.) Ch. II.5, pp. 107–20 (Elsevier Science, New York, 1998); andYamada, et al., J. Neurosurg. 86: 1012–17 (1997).

(28) Ocular Cell Implants

Two monolayers of cells, known as retinal pigment epithelium and cornealendothelium are essential for normal vision. In age-related maculardegeneration, the function of the retinal pigment epithelium is believedto be altered leading to visual loss. Replacement of this alteredepithelium with a healthy retinal pigment epithelium can potentiallyprovide a treatment for this debilitating condition. Transplantation ofdonor cell suspensions has been attempted but is problematic, and haslead to several attempts to use synthetic bioabsorbable polymers andprotein polymers as tissue engineering scaffolds to deliver retinalpigment epithelium and corneal endothelium into the eye.Polyhydroxyalkanoates can be used as scaffolds to deliver these cells,and monolayers derived therefrom, into the eye. They can be processedinto suitably sized scaffolds (specifically very thin yet strongconstructs), and do not produce acidic byproducts, like some of thecommercially available bioabsorbable synthetic polymers, which can bedeleterious to cell viability and function. Furthermore thepolyhydroxyalkanoate materials can be fabricated into appropriatescaffold devices with desirable mechanical and handling properties.Suitable methods to prepare polyhydroxyalkanoate ocular cell implantdevices are described in Hadlock et al., Tissue Engineering, 5:187–96(1999), and additional methods to produce other suitable tissueengineering scaffolds are described in Widmer & Mikos, “Fabrication ofbiodegradable polymer scaffolds for tissue engineering” in Frontiers inTissue Engineering (Patrick, et al., Eds.) Ch. II.5, pp. 107–20(Elsevier Science, New York, 1998); and, Yamada, et al., J. Neurosurg.86:1012–17 (1997).

(29) Spinal Fusion Cages

Spinal fusion cages are used to treat various forms of degenerative discdisease, a condition in which the spinal discs, located between eachvertebra, are no longer able to cushion and protect the vertebra duringmovement. This can result in severe, and occasionally, crippling backpain, as the vertebrae rub against adjacent spinal nerves. The conditionresults from the wearing down of the shock absorbing cartilage thatseparates the vertebrae of the spine, and can be due to aging or injury.Degenerating discs also become dehydrated losing height, and therebybringing the vertebrae closer together.

Degenerative disc disease can be treated surgically after othertherapies fail to provide relief. Surgical procedures known asdiscectomy or laminectomy are sometimes employed to remove the tissuethat is causing pain. The ultimate surgery is spinal fusion, wherein theaffected area of the vertebrae is disabled or immobilized eliminatingthe movement that is responsible for causing the pain. The traditionalspinal fusion procedure used to involve the use of bone graft material,with or without pedicle screws and plates, to fuse adjacent vertebraetogether. However, this procedure is traumatic, causes significantmuscle damage, meaningful loss of blood, and a long, sometimes painful,recovery period. Increasingly surgeons are using a relatively newprocedure involving spinal fusion cages to fuse two or more vertebraeinto one stable bony mass. In this procedure, a cage which comprises ahollow cylinder is implanted in the disc space, following removal of thedefective disc, and packed with bone graft material. Fusion occurs asnew bone grows into the fusion cages through holes in the cylinder. Thecages also serve to restore disc space height while the spine heals.Typically, a surgeon may employ two cages side by side in a procedure,and importantly, the procedure can be performed through small incisionseither through the front or back of the patient. The procedure has greatbenefits, allowing the surgeon a way to avoid cutting important backmuscles and having to reposition the delicate spinal chord. Recoveryrates are faster, better fusions and outcomes are achieved, and lessblood loss occurs during the procedure.

Polyhydroxyalkanoates can be fabricated into spinal fusion cages or cageparts using conventional processing techniques such as melt, solvent,and fiber processing. The advantages of using polyhydroxyalkanoates inthis application would be their ability to serve as transitionalconstructs providing the initial stability required prior to theformation of a stable fusion, yet bioabsorbing when they are no longerneeded—eliminating the presence and potential dangers of a foreignobject in the body. Resorption of the polyhydroxyalkanoate also wouldensure that full weight bearing is transferred to the spine and fusionsite, helping to prevent any subsequent resorption of bone, loss ofstrength, instability, or movement of the fusion device. This can beachieved either by making a spinal cage completely frompolyhydroxyalkanoate polymers, blends, or composites of other materials,or by incorporating into such a device a polyhydroxyalkanoate componentthat transfers stress and strain away from the fusion cage and onto thespine as the component bioabsorbs. The component can be, for example, arod, washer, screw, pin, strut, plate, staple, or a combination of suchelements. Devices can also be fabricated from polyhydroxyalkanoateswhich would be expected to provide improved results, particularly bypromoting new bone growth formation. These devices could incorporatefusion promoting substances in the polymer which is not readily achievedwith the current metal fusion cages and devices. The polymers can alsobe configured in porous and non-porous forms. Designs and methods whichcan be used to prepare polyhydroxyalkanoate spinal cages are disclosedin U.S. Pat. Nos. 5,895,426; 4,936,848; 4,961,740; 5,910,315; 5,645,598;4,743,236; 5,665,122; and 5,910,315.

(30) Wound Dressing Agents and Hemostats

Polyhydroxyalkanoates can be used to fabricate wound dressings andhemostat devices. There are several properties that dressing materialsfor wounds ideally should possess, including an ability to remove excessexudate from the wound, protect the wound from mechanical injury, andreduce the risk of infection. The wound dressing must be free of toxicsubstances, and it should not adhere to the wound which would disturbthe healing process. Commonly used dressings include cellulosicdressings such as cotton lint, cotton gauze, cotton wool pads,cotton/rayon wool pads faced with non-woven materials. Other dressingscontain polyurethanes, polyurethane-polyols, and/or naturalpolysaccharide or protein polymers such as collagen. These dressings maybe impregnated, coated, or otherwise contain agents such as alginateswhich raise the absorptive capacity of the dressing and can stimulatethe clotting cascade for bleeding wounds, and/or other agents such assilver salts, antiseptics, analgesics, and/or preservatives. Thedressings may be prepared, for example, as fiber mats, sponges, foams,nets, fibrous substrates. The dressings can be prepared to have a rangeof different pore sizes and densities. The dressings can be used in thetreatment of a variety of wound types, including pressure sores,decubitus ulcers, venous stasis ulcers, infected wounds, deep and opensurgical wounds and incisions, sealing of percutaneous incisions orpunctures, and burns.

The advantages of using the polyhydroxyalkanoate polymers in these wounddressing and hemostat applications include the ability to provide amicroclimate, and/or a tissue scaffold, for tissue regeneration. It ispossible to produce wound dressings and hemostats that bioabsorb invivo, because the polyhydroxyalkanoates are bioabsorbable.Alternatively, non-absorbable dressings, particularly for externalapplication, can be prepared. Wound dressings may be prepared frompolyhydroxyalkanoate polymers that are comfortable, flexible, andabsorbent. They may be prepared, for example, as fiber mats, sponges,foams, nets, fibrous or porous forms, and can have a range of pore sizesand densities. The PHA wound dressings and hemostats also can beprepared to include other agents such as alginates, silver salts,antiseptics, analgesics, and preservatives. The hydrophobicity,hydrophilicity, absorption capacity, surface properties, and mechanicalproperties of the wound dress or hemostat can be modified, for example,by varying the nature of the monomer hydroxy acids incorporated into thepolymer. It is also possible to incorporate polyols into thepolyhydroxyalkanoate polymers to change these properties. Such methodsare described, for example, in PCT WO 97/07153 and U.S. Pat. No.5,994,478. The polyhydroxyalkanoates also may be used as a component ofa wound dressing or hemostat device, for example, with a polyurethane orcollagen material. Examples of suitable methods for preparing wounddressing devices and hemostats are described in U.S. Pat. Nos.3,978,855; 4,664,662; 5,021,059; 5,676,689; 5,861,004; and 5,914,125.

(31) Bone Dowel Devices

Polyhydroxyalkanoates can be fashioned into dowels for spinal or otherorthopedic repair. Bone dowels are commonly used in spinal fusionprocedures for a variety of reasons, for example, to treat patients withdegenerative disc disease, deformities, as well as those involved intraumatic injuries. In posterior fusions, bone is typically removed fromthe hip area and placed in a traverse direction between adjacentvertebrae, often with the aid of spinal instruments. The instrumenthelps to hold the spine together so that a bone fusion can occur.Fusions in the lumbar area also can be done anteriorly, wherein the discis removed and bone graft is placed between the two adjacent vertebralbodies. Other procedures employ a threaded bone dowel which is typicallya piece of cadaver donor bone that has been machined into a screwconfiguration and can be hollowed out to form a cage. The bone dowel isthen filled with a small amount of bone from the patient's hip. Atapping device is then used to create screw threads in the vertebralbodies that will be fused by the bone dowel. The bone dowel is thenscrewed into place between the vertebrae.

Polyhydroxyalkanoates can be made into these dowels, which can take theform of hollowed constructs able to receive bone graft and be placedbetween vertebrae, as well as composite dowel constructs. The keyadvantages of using polyhydroxyalkanoates to construct these devices istheir ability to provide the transitional support (e.g., mechanicalsupport) necessary during formation of a stable fusion, coupled withtheir ability to completely resorb, thereby transferring full weightbearing to the spine in a manner able to prevent, or at least minimize,loss of bone mass and strength, and prevent subsequent movement in thefusion area. The polyhydroxyalkanoate bone dowels can be used or withoutadditional hardware, or can be incorporated into such hardware,particularly in a manner that allows weight bearing to be subsequentlytransferred from the hardware to the spine upon resorption of thepolyhydroxyalkanoate component. The dowels can be formed, for example,by using conventional polymer processing techniques with molds and/ormachining methods. The dowels may be threaded, porous or non-porous, asdesired. If necessary, x-rays and CT scans can be used in thefabrication process to custom make the dowel for patients. Examples ofbone dowels and applications therefor are described in U.S. Pat. Nos.4,501,269; 5,015,255; 5,522,894; and 5,860,973.

The polyhydroxyalkanoates also can be used to improve thebiocompatibility of other devices and tissue engineering constructs. Forexample, a polyhydroxyalkanoate coating can be coated onto devices orconstructs formed of less biocompatible materials.

(32) Heart Values and Vascular Grafts

The unidirectional flow of blood through the entire circulatory systemis controlled by the heart's valves. Humans have a total of four heartvalves: the tricuspid valve, the pulmonary valve, the mitral valve, andthe aortic valve. With the exception of the mitral valve which has justtwo cusps (or leaflets), each valve has three cusps which are forcedopen and shut by differences in pressure within the heart. Valvularheart disease, which is characterized by a defective heart valve,impairs the ability of the heart to function properly. This can becaused by degenerative processes, congenital defects, bacterialendocarditis, or rheumatic fever, and results in oscillations of apatient's blood pressure and circulation, leading to heart murmurs,heart failure, or myocardial infarction (insufficient blood flow toheart muscle).

Currently, there are a number of different methodologies employed totreat heart valve disease, including drug treatments, valve repair andvalve replacement. In non-life threatening situations, drugs used in thetreatment of congestive heart failure are usually employed first to makethe heart work harder and pump blood throughout the body. However, oncevalvular disease progresses to the point at which the heart's ability topump blood is significantly impaired, surgery is usually recommended torepair or replace the diseased valve. Many surgeons prefer to repair aheart valve when possible; however, in many cases this is either notpossible or the benefits are short lived.

Valvular replacement surgery is a traumatic procedure which involvesplacing a patient on cardiopulmonary bypass while the diseased valve isreplaced with an artificial valve prosthesis. There are currently twoprimary types of artificial valve prostheses: mechanical heart valvesand tissue heart valves. Each type has benefits and drawbacks.Mechanical valves, for example, are noted for their durability andreliability. However, a major drawback is the need for the recipient tobe placed upon a lifelong anticoagulant therapy which involvescontinuous monitoring of anticoagulant levels. Current tissue valves,derived from heterologous sources (cows and pigs), on the other-hand, donot require anticoagulant therapy, they are quiet, provide physiologicalflow patterns, and typically have slowly developing rather thancatastrophic failure modes. The major problem associated with thesevalves is their lack of durability. Most of the current tissue valvesgenerally last between five and fifteen years before they need to bereplaced due to a gradual deterioration of the (non-living) tissue.

Most experts agree that if the durability problem can be solved, tissuevalves would be the clear choice for treatment of valvular heartdisease, as no synthetic material has proven to have the propertiesneeded to endure bi-directional flexing some 40 million times a yearwithout producing thrombosis. Furthermore, mechanical valves cannot beused to repair valve leaflets. One potential solution which couldaddress the deficiencies of current valve replacements is to develop atissue engineered heart valve. The valve would initially comprise aheart valve scaffold material which could be seeded with appropriatecells, implanted, and serve as a transitional construct which isabsorbed leaving an entirely new living tissue heart valve in vivo. Inthe approach, the tissue engineered heart valve can be seeded andimmediately implanted, or seeded and cultured in a bioreactor beforeimplantation. In the latter instance tissue formation and polymerbioabsorption can be complete before implantation or preferably continueafter implantation. The advantages of developing tissue engineered heartvalves would be several fold. First, the ultimate product would be adurable living heart valve able to withstand the demands of the body. Itcan be derived from non-immunogenic tissue obviating the need foranticoagulant therapy, furthermore, the tissue can be derived from anautologous source virtually eliminating the risk of diseasetransmission. In the case of infants and children where growth is aconcern, the use of a living tissue valve would remove the need toreplace the valve as the patient grows. Finally, in cases where repairrather than replacement is preferable, the tissue engineering solutionwould potentially provide a source of suitable living tissue.

Tissue engineered heart valves, and components of heart valves such asleaflets or supports, derived from polyhydroxyalkanoate polymers, whichoffer the necessary mechanical properties and bioabsorption profiles,may be produced by constructing porous heart valve scaffolds from thesepolymers alone or with other materials. Preferably, these scaffolds arederived from foams and/or fibrous polyhydroxyalkanaote polymers. Thescaffolds, if desired, may be coated with other agents such as proteinsto facilitate cell attachment and proliferation. The scaffolds are thensubsequently seeded with the appropriate cells. Suitable cells includecardiovascular and vascular cells, smooth muscle cells, endothelialcells, as well as stem cells. Ideally, the cells are autologous butother non-immunogenic options are also acceptable. The seeded constructmay then be incubated in a bioreactor prior to implantation. Preferably,the bioreactor subjects the heart valve to changes in flow and pressure,essentially mimicking in vivo conditions. A pulsatile bioreactor ispreferred. At any time after seeding, the seeded construct may beimplanted in vivo. Preferably, this is one to two weeks after seeding.Methods illustrative of the approach are described by Breuer, et al.Biotechnology & Bioengineering, 50:562–67 (1996); Shinoka, et al., Ann.Thorac. Surg 60:S513–6, (1995); Zund et al., Euro. J. Cardio-thoracicSurgery 11:493–97 (1997).

Vascular Grafts

Vascular grafts are currently inserted to repair or replace compromisedblood vessels, in the arterial or venous systems, that have been subjectto damage or disease such as atherosclerosis, aneurysmal disease, andtraumatic injury. Currently, there are three grafting optionst namely,an autograft, a synthetic graft, or a cryopreserved graft when anautograft is not available. The choice between an autograft and asynthetic graft depends upon a number of factors. In general, syntheticgrafts are restricted to applications involving the replacement of largeand medium size vessels. Typically, these synthetic vessels remain opento blood flow for around 5 years before they begin to fail. Smallerdiameter synthetic grafts, however, where blood flow rates are lower,generally fail rapidly, and thus are not used in procedures such ascoronary artery bypass grafting (CABG), the most common open heartsurgical procedure requiring smaller diameter vessels. When syntheticvascular grafts cannot be used (as in CABG), the preferred procedureinvolves the use of an autograft, which entails a second traumaticsurgical procedure to harvest a suitable artery or vein from thepatient. In some cases, the harvested vessels can be unsuitable for use,and in other cases there can be a shortage of harvestable autograftsparticularly if the patient has previously had the same operation. [Ithas been estimated that 40% of CABG patients receiving saphenous veinbypasses will require subsequent intervention within ten years of theoriginal operation (Vector Securities International, ThoratecLaboratories Company Report, November 1997)]. For these reasons, thereis a strong need to develop a vascular graft particularly for CABGprocedures, and below the knee grafting procedures, which will remainopen to blood flow, as well as larger diameter grafts to improve patencyrates. Tissue engineered vascular graft, comprising cell seeded vascularscaffolds, which can be derived from polyhydroxyalkanoate polymers,offer such a solution to these problems. These polymers offer a suitablecombination, either alone or with other materials, of bioabsorptionrates and mechanical properties. Tissue engineered polyhydroxyalkanoatederived vascular grafts can be produced by forming a tubular constructof the appropriate diameter (typically 3–10 mm internal diameter) andseeding this construct with appropriate cells. Ideally, thepolyhydroxyalkanoate is porous in nature, and the construct can belaminated. (In a variation of this approach, a non-cylindrical constructmay be seeded and subsequently formed into a tubular construct.) Theseeded tubular construct can be implanted directly, or preferablyincubated in a bioreactor prior to implantation. Preferably, thebioreactor is capable of subjecting the construct to conditions similarto those experience in vivo, namely pulsatile flow. The cells areideally autologous, and/or non-immunogenic. Suitable cells includecardiovascular cells, vascular cells, endothelial cells, smooth musclecells, as well as stem cells. Methods illustrative of the approach aredescribed by Shinoka, J. Thoracic & Cardiovascular Surgery, 115:536–546,Niklason, Science, 284:489–493 (1999).IV. Methods of Fabricating the Devices

Preferred methods of fabricating medical devices include solventcasting, melt processing, extrusion, injection and compression molding,fiber firming, and spray drying. Particles are preferably prepareddirectly from a fermentation based process, or by a solvent evaporationtechnique, double emulsion technique, or by microfluidization, usingmethods available in the art. (Koosha, Ph.D. Dissertation, 1989, Univ.Nottingham, UK., Diss. Abstr. Int. B 51:1206 (1990); Bruhn & Müeller,Proceed. Intern. Symp. Control. Rel. Bioact. Mater. 18:668–69 (1991);Conti, et al., J. Microencapsulation, 9:153–66 (1992); Ogawa, et al.,Chem. Pharm. Bull., 36:1095–103 (1988); Mathiowitz & Langer,“Polyanhydride microspheres as drug delivery systems,” in MicrocapsulesNanopart. Med. Pharm. (Donbrow, Ed.) ch. 5, pp. 99–123 (CRC, Boca Raton,Fla. 1992)).

The PHAs can be fabricated into devices suitable for wound healing. Forexample, non-woven fibrous materials for this purpose may be preparedfrom the polymers by first producing polymer fibers, by pressing thepolymers through a perforated outlet, using procedures known to thoseskilled in the art. The fibers can then be fabricated into a porousmembrane (cloth) by spreading them on a solid support and subjectingthem to compression molding. The thickness of the device is preferablyless than 500 μm. The wound healing device may also be prepared byperforating a film or membrane using a laser to achieve porosity, orusing a leaching technique to prepare a porous material. The pore sizesshould ideally be small enough to lock out cells and other tissuematter. The wound healing devices may be positioned in vivo to separatetissues and stimulate tissue regeneration.

The PHAs may be used to encapsulate cells. Using procedures known tothose skilled in the art, cells first may be pre-coated (see Maysinger,Reviews in the Neurosciences, 6:15–33 (1995)). Using a particleencapsulation procedure such as the double emulsion technique, the cellsmay then be encapsulated by PHAs. Ogawa, et al., Chem. Pharm. Bull.,36:1095–103 (1988). Encapsulated cells may then be implanted in vivo.

The PHAs may be fabricated into tissue engineering scaffolds using awide range of polymer processing techniques. Preferred methods offabricating PHA tissue engineering scaffolds include solvent casting,melt processing, fiber processing/spinning/weaving, or other means offiber forming extrusion, injection and compression molding, lamination,and solvent leaching/solvent casting. Such methods are known to thoseskilled in the art.

One preferred method of fabricating a PHA tissue engineering scaffoldinvolves using an extruder, such as a Brabender extruder. For example,this technique can be used to prepare extruded tubes suitable forimplantation in a range of lengths and sizes.

Another preferred method involves preparing a nonwoven PHA scaffold fromfibers. Fibers may be produced from the melt or solution, and processedinto nonwovens using methods known to those skilled in the art. Theproperties of the nonwoven may be tailored by varying, for example, thePHA material, the fiber dimensions, fiber density, material thickness,fiber orientation, and method of fiber processing. The porous membranesmay, if desired, be further processed. For example, these membranes maybe formed into hollow tubes.

Another preferred method involves melt or solvent processing a suitablePHA into an appropriate mold and perforating the material using a laseror other means to achieve the desired porosity. Also preferred aremethods that include rolling a compression molded PHA sheet into a loopand heat sealing. The PHA sheet optionally may be rolled with anothermaterial, such as a second biodegradable polymer. For example, thelatter material can be a nonwoven of polyglycolic acid, polylactic acid,or a copolymer of glycolic and lactic acids, providing, for example, alaminated tube suitable for use in the engineering of new vessels,ducts, and tubes. The PHAs may also be used to coat other tissueengineering scaffolds. Such materials could be derived from otherdegradable polymers. Coating may be performed, for example, with asolvent based solution, or by melt techniques, or using a PHA latex.

The tissue engineering devices described herein may be seeded with cellsprior to implantation or after implantation. The cells may be harvestedfrom a healthy section of the donor's tissue, expanded in vitro usingcell culture techniques, and then seeded into a scaffold (or matrix)either prior to or after implantation. Alternatively, the cells may beobtained from other donor's tissue or from existing cell lines.

The PHAs may be used to coat other devices and materials. Such coatingsmay improve their properties for medical application, for example,improving their biocompatibility, mechanical properties, and tailoringtheir degradation and controlled release profiles. The PHAs may becoated onto other devices using the fabrication procedures describedabove. The thickness of the coating can be adjusted to the needs of thespecific application by changing the coating weight or concentrationapplied, and/or by overcoating.

The PHAs may be fabricated into stents using a wide range of polymerprocessing techniques. Preferred methods of fabricating PHA stentsinclude solvent casting, melt processing, fiber processing/spinning,extrusion, laser ablation, injection molding, and compression molding.Such methods are known to those skilled in the art.

Methods for manufacturing the devices which increase porosity or exposedsurface area can be used to alter degradability. For example, asdemonstrated by the examples, porous polyhydroxyalkanoates can be madeusing methods that creates pores, voids, or interstitial spacing, suchas an emulsion or spray drying technique, or which incorporate leachableor lyophilizable particles within the polymer.

Additional methods for fabricating the polyhydroxyalkanoate devices aredescribed in Biomaterials Science (Ratner, et al., Eds.) Academic Press,San Diego, Calif. 1996; Biomedical Applications of Polymeric Materials(Tsuruta, et al., Eds.) CRC Press, Boca Raton, Fla., 1993; SyntheticBiodegradable Polymer Scaffolds (Atala, et al., Eds.) Birhauser, Boston,1997; Wound Closure Biomaterials and Devices, (Chu, Jet al., Eds.) CRCPress, Boca Raton, Fla., 1997; Polyurethanes in Biomedical Applications(Lamba, et al., Eds.) CRC Press, Boca Raton, Fla., 1998; Handbook ofBiodegradable Polymers (Domb, et al., Eds.) Harwood Academic Publishers,Amsterdam, The Netherlands, 1997.

V. Using the Devices and Composition

The polyhydroxyalkanoate devices (including coatings) and composition,can be delivered by any means including open surgery or by a minimallyinvasive method such as ingestion or injection or insertion.Furthermore, depending upon the application, the composition may befurther modified to include other materials, such as bioactive agentslike growth factors, drugs, antimicrobial agents, angiogenesis factors,or materials that modify the properties of the device such as otherpolymers, plasticizers, nucleants, and fillers.

When the depyrogenated PHAs are implanted in the body, these materialsshow very little, if any, acute inflammatory reaction or any adversetissue reaction. There is no significant inflammatory response or scartissue formation. Recruitment of inflammatory cells is minimal.Histological examination of the explanted devices demonstrates that thematerials are essentially inert. Accordingly, devices constructed ofPHAs can be implanted with minimal adverse affect on the surroundingtissue. Release of the hydroxy acid degradation products from theimplanted materials typically is slow and well tolerated by the body.Thus, PHAs are expected to maintain their material properties for amatter of months and will eventually degrade to non-toxic materials.

Devices prepared from the PHAs can be used for a wide range of differentmedical applications. Examples of such applications include controlledrelease, drug delivery, tissue engineering scaffolds, cellencapsulation, targeted delivery, biocompatible coatings, biocompatibleimplants, guided tissue regeneration, wound dressings, orthopedicdevices, prosthetics and bone cements (including adhesives and/orstructural fillers), and diagnostics.

The PHAs can encapsulate, be mixed with, or be ionically or covalentlycoupled to any of a variety of therapeutic, prophylactic or diagnosticagents. A wide variety of biologically active materials can beencapsulated or incorporated, either for delivery to a site by thepolyhydroxyalkanoate, or to impart properties to the polymer, such asbioadhesion, cell attachment, enhancement of cell growth, inhibition ofbacterial growth, and prevention of clot formation.

Examples of suitable therapeutic and prophylactic agents includesynthetic inorganic and organic compounds, proteins and peptides,polysaccharides and other sugars, lipids, and DNA and RNA nucleic acidsequences having therapeutic, prophylactic or diagnostic activities.Nucleic acid sequences include genes, antisense molecules which bind tocomplementary DNA to inhibit transcription, and ribozymes. Compoundswith a wide range of molecular weight can be encapsulated, for example,between 100 and 500,000 grams or more per mole. Examples of suitablematerials include proteins such as antibodies, receptor ligands, andenzymes, peptides such as adhesion peptides, saccharides andpolysaccharides, synthetic organic or inorganic drugs, and nucleicacids. Examples of materials which can be encapsulated include enzymes,blood clotting factors, inhibitors or clot dissolving agents such asstreptokinase and tissue plasminogen activator; antigens forimmunization; hormones and growth factors; polysaccharides such asheparin; oligonucleotides such as antisense oligonucleotides andribozymes and retroviral vectors for use in gene therapy. The polymercan also be used to encapsulate cells and tissues. Representativediagnostic agents are agents detectable by x-ray, fluorescence, magneticresonance imaging, radioactivity, ultrasound, computer tomagraphy (CT)and positron emission tomagraphy (PET). Ultrasound diagnostic agents aretypically a gas such as air, oxygen or perfluorocarbons.

In the case of controlled release, a wide range of different bioactivecompounds can be incorporated into a controlled release device. Theseinclude hydrophobic, hydrophilic, and high molecular weightmacromolecules such as proteins. The bioactive compound can beincorporated into the PHAs in a percent loading of between 0.1% and 70%by weight, more preferably between 5% and 50% by weight. The PHAs may bein almost any physical form, such as a powder, film, molded item,particles, spheres, latexes, and crystalline or amorphous materials.They can be combined with additional non-PHA materials, for example,other polymers. They are suitable for use in applications requiringslowly degrading, biocompatible, moldable materials, for example,medical devices. Examples of medical devices which can be prepared fromthe polymers include rods, bone screws, pins, surgical sutures, stents,tissue engineering devices, drug delivery devices, wound dressings, andpatches such as hernial patches and pericardial patches.

Degradable implants fabricated with the PHAs may be used in a wide rangeof orthopedic and vascular applications, tissue engineering, guidedtissue regeneration, and applications currently served by otherthermoplastic elastomers (McMillin, Rubber Chem. Technol, 67:417–46(1994)). The implants may include other factors to stimulate repair andhealing. Preferred devices are tubes suitable for passage of bodilyfluids. These devices may be modified with cell attachment factors,growth factors, peptides, and antibodies and their fragments.

Prior to implantation, a bioresorbable polymeric article must besterilized to prevent disease and infection of the recipient.Sterilization is performed prior to seeding a polymeric device withcells. Heat sterilization of PHA containing articles is oftenimpractical since the heat treatment could deform the article,especially if the PHA has a melting temperature below that required forthe heat sterilization treatment. This problem can be overcome usingcold ethylene oxide gas as a sterilizing agent. Exposure of a PHAcontaining article to vapors of ethylene oxide prior to implantationsterilizes the article making it suitable for implantation. Duringsterilization with cold ethylene oxide gas, gamma-irradiation, the PHAcontaining article maintains its shape. This type of treatment isideally suited for sterilization of molded, or pre-formed articles wherethe shape of the article plays in important role in its properfunctioning.

The devices described herein can be administered systemically orlocally, or even used in vitro, particularly for cell culture. Thepreferred methods of systemically administering the devices are byinjection, inhalation, oral administration and implantation. Othersuitable methods for administering the devices include administering thedevices topically, as a lotion, ointment, patch, or dressing.

The compositions and methods described herein will be further understoodwith reference to the following non-limiting examples.

EXAMPLE 1 Production of P4HB in Recombinant E. coli

E. coli strain MBX 1177, a derivative of strain DH5α selected for theability to grow with 4-hydroxybutyric acid (4HB) as the sole carbonsource, was transformed with pFS30, a plasmid containing the genesencoding PHA synthase from Ralstonia eutropha, 4-hydroxybutyryl-CoAtransferase from Clostridium kluyveri, and β-lactamase, which confersresistance to ampicillin. The synthase and transferase are under thecontrol of the trc promoter, which is inducible byisopropyl-β-D-thiogalactopyranoside (IPTG) in pFS30. These cells werefirst grown in 100 ml LB (Luria Broth, Difco, Detroit, Mich.; 25 g/L)plus 100 μg/ml ampicillin overnight in a 250-ml Erlenmeyer flask at 37°C. with shaking at 200 rpm. This entire culture was used as an inoculumfor the fermentation carried out in a 7L vessel. The first stage of thefermentation consisted of growing biomass in 5 L of LB-ampicillin at 37°C. with stirring at 800 rpm and aeration at 1 volumetric volume ofair/min (vvm). After 17 hours, the volume was adjusted to 6 L by addingone liter of medium, such that the total volume contained, per liter:2.5 g LB powder, 5 g 4HB as sodium salt, 2 g glucose, 50 mmol potassiumphosphate (pH 7), 7 g phosphoric acid, 100 μg ampicillin, and 0.1 mmolIPTG. At this time, the temperature was adjusted to 33° C., and theagitation rate was reduced to 400 rpm. Periodic additions of glucose andsodium 4HB were made when the pH was significantly below or above 7,respectively, because the addition of glucose caused the pH to decreaseslowly and the addition of 4HB caused the pH to increase slowly. The pHwas not automatically controlled. The fermentation proceeded this wayfor an additional 70 h, at which time a total of 34 g/L glucose and 15g/L 4HB had been fed. The cells were allowed to settle at 4° C. for 2days, after which time the liquid phase was pumped away, and the cellslurry was fluidized in a Microfluidics Corporation (Newton, Mass.)M110-EH Microfluidizer at 18,000 psi. The resulting material waslyophilized and extracted into tetrahydrofuran (THF, 3% wt/vol P4HB)with heating (60° C.) and mechanical stirring. The resulting THF extractwas pressure filtered through glass micro-fiber (2.3 μm) and Teflon (2μm) depth filters. The polymer was precipitated into an equal volume ofwater and lyophilized. The polymer was redissolved in THF (3%, wt/volP4HB) with heating (60° C.) and the solution was filtered through glassmicro-fiber (2.3 μm) and Teflon (2 μm) depth filters and precipitatedinto water/THF (1:1). The precipitate was washed with water/THF (1:1)and lyophilize to yield a white colored foam (20 g). This material wasidentified as poly-4-hydroxybutyrate and shown to be non-cytotoxic by anagar diffusion assay (ISO 10993, Toxicon Corp., Bedford, Mass.).Elemental analysis was C 55.63%, H 7.41%, O 37.28%, N 41 ppm. GCanalysis shows very low lipids in the purified polymer. NMR analysisshows expected peaks and no lipids.

EXAMPLE 2 Production of Poly(4HB-co-2HB) in Recombinant E. coli

E. Coli strains MBX1177/pFS30 and MBX184 (CGSC6966)/pFS30 wereprecultured in 300 mL LB-ampicillin in a one-liter Erlenmeyer flask at30° C. overnight with shaking at 200 rpm. Two 100-mL aliquots of eachpreculture were centrifuged (2000×g, 10 minutes), and the cells obtainedfrom each of these aliquots were resuspended in 100 mL of a mediumcontaining, per liter: 6.25 g LB powder; 2 g glucose; 50 mmol potassiumphosphate (pH 7); 100 μg ampicillin; and 100 μmol IPTG. The medium alsocontained 2-hydroxybutyric acid (2HB) and 4HB; in one flask theconcentrations were 8 g/L 2HB and 2 g/L 4HB, and in the other theconcentrations of the two acids were each 5 g/L. Both acids were addedto the flasks as the sodium salt; the masses given for the acids do notinclude the mass of sodium. These four flasks (two flasks for eachstrain) were incubated at 30° C. for an additional 48 hours with shakingat 200 rpm. The cells were removed from the medium by centrifugation(2000×g, 10 minutes), washed once with water, centrifuged again, andlyophilized. Gas chromatographic analysis was carried out on thelyophilized cell mass to analyze for polymer content and composition.The cellular contents and compositions of the PHAs produced are shown inTable 2: When the ratio of 2HB to 4HB was 4:1, the 2HB content of thepolymer was higher than 19 percent for both strains by GC analysis,while at a 1:1 ratio of 2HB to 4HB, the 2HB content of the polymer wasaround 1 percent. The 4HB was more readily incorporated into the polymerthan was the 2HB; therefore, when 4HB was present at 2 g/L the overallpolymer content of the cells is less than when it was present at 5 g/L.The polymers produced by MBX184/pFS30 were extracted from the cells andanalyzed. The lyophilized cell mass was incubated in 5 mL of chloroformat 37° C. for 2 hours. The cell debris was removed by centrifugation(2000×g, 5 minutes), and the resulting polymer solution was addeddropwise to 50 mL of ethanol to precipitate it. The precipitated polymerwas centrifuged from the ethanol as described above. In the case of the4:1 2HB:4HB ratio, the polymer was difficult to centrifuge from theethanol; it formed a haze when added to the ethanol, but not nearly allof it could be collected by centrifugation, probably because themolecular weight of this polymer was rather low. The polymer isolatedfrom the 1:1 2HB:4HB flask was easily precipitated from the ethanol, andit was recovered nearly completely. GC analysis of these extractedsamples (Table 2) show that the 2HB content was slightly lower than whenthe analysis was done on whole cells. It is possible that 2HB residuesin the polymer chain are hydrolyzed during the extraction, thus loweringthe apparent 2HB content in the extracted samples. The fact that themolecular weight of the extracted polymer was apparently lower when the2HB content was higher is consistent with this explanation.

A second experiment was performed with MBX184/pFS30. These cells wereprecultured in 400 mL LB-ampicillin in a one-liter Erlenmeyer flask at30° C. overnight with shaking at 200 rpm. An addition of 20 ml of mediumwas made to each flask such that the total volume contained, per liter:2.5 g additional LB powder; 2 g 4HB as sodium salt; 2 g glucose; 50 mmolpotassium phosphate (pH 7); 100 μg ampicillin; 50 μmol IPTG; and 2, 4,6, or 8 g 2HB as sodium salt. The flasks were incubated for anadditional 48 hours at 30° C. and 200 rpm. The cells were removed fromthe medium by centrifugation (2000×g, 10 minutes), washed once withwater, centrifuged again, and lyophilized. The dried cell mass wassubjected to GC analysis as described above. Table 3 shows the cellcontent and composition of the polymers thus obtained. At low 2HB:4HBratios, little or no 2HB was incorporated into the polymer; however,when this ratio was 3:1 or 4:1, 2HB incorporation into the polymer wasvery significant. The overall polymer content of all the cells wasrather low, probably because the acids are not present at concentrationshigh enough to permit the uptake and/or incorporation to proceed at ahigh rate.

TABLE 2 GC Analysis of Poly(4HB-co-2HB) From MBX1177/pFS30 andMBX184/pFS30 4HB, 2HB, Total PHA, P4HB, P2HB, Strain g/L g/L % ofdcw^(a) % of PHA^(b) % of PHA^(b)  184/30 2 8 18.3 70.8 19.2 (14.2)^(c) 184/30 5 5 47.1 98.8  1.2 (0.9)^(c) 1177/30 2 8 13.0 62.3 27.7 1177/305 5 40.1 98.9  1.1 ^(a)dcw: dry cell weight. ^(b)Determined by GCanalysis. About 20 mg of lyophilized cell mass was subjected tobutanolysis at 110° C. for 3 hours in 2 mL of a mixture containing (byvolume) 90% 1-butanol and 10% concentrated hydrochloric acid, with 2mg/mL benzoic acid added as an internal standard. The water-solublecomponents of the resulting mixture were removed by extraction with 3 mLwater. The organic phase (1 μL at a split ratio of 1:50 at an overallflow rate of 2 mL/min) was analyzed on an SPB-1 fused silica capillaryGC column (30 m; 0.32 mm ID; 0.25 μm film; Supelco; Bellefonte, Pa.)with the following temperature profile: 80° C., 2 min.; 10° C. per min.to 250° C.; 250° C., 2 min. The standard used to test for the presenceof 4-hydroxybutyrate units in the polymer was γ-butyrolactone. Thestandard used to test for 2-hydroxybutyrate units in the polymer wassodium (2-hydroxybutyrate). ^(c)Percentages in parentheses weredetermined by GC analysis as above, but after extraction of the polymerinto chloroform and subsequent precipitation in ethanol.

TABLE 3 GC Analysis of Poly(4HB-co-2HB) From MBX184/pFS30 4HB, 2HB,Total PHA, P4HB, P2HB, Sample g/L g/L % of dcw^(a) % of PHA^(b) % ofPHA^(b) 1 2 2 8.2 100 0 2 2 4 5.6 100 0 3 2 6 5.7 84.1 15.9 4 2 8 4.154.3 45.7 ^(a)dcw: dry cell weight. ^(b)Determined by GC analysis. SeeTable 2 for details.

EXAMPLE 3 Production of poly(4HB-co-3HB) in Recombinant E. coli

Strain MBX1177/pFS30 was precultured in 100 ml LB-ampicillin in fourseparate 250-ml Erlenmeyer flasks at 30° C. overnight with shaking at200 rpm. An addition of 20 ml of medium was made to each flask such thatthe total volume contained, per liter: 2.5 g additional LB powder; 4 g4HB as sodium salt; 4 g glucose; 50 mmol potassium phosphate (pH 7); 100μg ampicillin; 50 μmol IPTG; and 0.25, 0.5, 0.75, or 1 g3-hydroxybutyrate (3HB) as sodium salt. The flasks were incubated for anadditional 48 hours at 30° C. and 200 rpm. The cells were removed fromthe medium by centrifugation (2000×g, 10 minutes), washed once withwater, centrifuged again, and lyophilized. Gas chromatographic analysiswas carried out on the lyophilized cell mass to analyze for polymercontent and composition. The standard used to test for 3-hydroxybutyrateunits in the polymer was poly(3-hydroxybutyrate). The cellular contentsand compositions of the PHAs produced are given in Table 4. As the ratioof 4HB/3HB in the medium decreased, the 3HB content of the polymerincreased in a monotonic fashion, while the overall polymer content ofthe cells was similar in all trials, which means that the composition ofthe medium can be used predictably to control the copolymer compositionwithout significantly affecting the overall polymer yield. The polymerwas extracted from the remainder of the lyophilized cell mass. For allsamples, lyophilized cell mass was mixed with about three times its ownvolume of 1,2-dichloroethane and incubated with mild shaking in a closedtube at 37° C. for 6 hours. The particulate matter was separated fromthe polymer solution by centrifugation (2000×g, 10 minutes). Theresulting solution was added dropwise to about 10 times its own volumeof ethanol, and the precipitated polymer was allowed to settle out ofsolution. The supernatant was poured off, and the remaining wet polymerwas allowed to stand until it appeared to be dry. The polymer was thenlyophilized to complete dryness. Thermal properties of these P4HB-co-3HBcompositions are shown in Table 5.

TABLE 4 GC Analysis of Poly(4HB-co-3HB) From MBX1177/pFS30 4HB, 3HB,Total PHA, P4HB, P3HB, Sample g/L g/L % of dcw^(a) % of PHA^(b) % ofPHA^(b) 3a 4 0.25 49.3 98.0 2.0 3b 4 0.5 46.7 94.2 5.8 3c 4 0.75 56.691.7 8.3 3d 4 1 51.8 89.4 10.6 ^(a)dcw: dry cell weight. ^(b)Determinedby GC analysis. See Table 2 for details. The standard used to test forthe presence of 4-hydroxybutyrate units in the polymer wasγ-butyrolactone. The standard used to test for 3-hydroxybutyrate unitsin the polymer was poly(3-hydroxybutyrate).

TABLE 5 Properties of P4HB and P4HB-co-3HB From MBX1177/pFS30 dH %^(a)%^(a) Tm^(b) Tm1^(b) Tg^(b) Tx^(b) Tm2^(b) Sample 4HB 3HB (° C.) (J/g)(° C.) (° C.) (° C.) Mw^(c) P4HB 100 0 60 45 −51 −16 X 1,000,000 3b 94.25.8 47 36 −52  −4 44 1,500,000 3c 91.7 8.3 40 20 −53 nd 39 1,900,000 3d89.4 10.6 39 17 −53 nd nd 1,100,000 nd = not detected. ^(a)Determined byGC analysis, see Table 2 for details. ^(b)Determined by DSC analysis. APerkin Elmer Pyris 1 differential scanning calorimeter was used. Samplesmasses were approximately 4–8 mg. The thermal program used was asfollows: 25° C., 2 min.; heat to 195° C. at 10° C. per min.; hold at195° C. 2 min.; cool to −80° C. at 300° C. per min.; hold at −80° C. for2 min.; heat to 195° C. at 10° C. per min. The melting temperature (Tm)and the enthalpy of fusion of this melting peak (dHTm1) were determinedin the first heating cycle. Glass transition temperature (Tg),crystallization temperature (Tx) and melting temperature (Tm2) weredetermined during the second heating cycle. ^(c)Determined by GPCanalysis. Isolated polymers were dissolved in chloroform atapproximately 1 mg/mL and samples (50 μL) were chromatographed on aWaters Stryagel HT6E column at a flow rate of 1 mL chloroform per minuteat room temperature using a refractive index detector. Molecular masseswere determined relative to polystyrene standards of narrowpolydispersity.

EXAMPLE 4 In Vitro and In Vivo Degradation of P4HB

The degradation of P4HB was studied in vitro and in vivo. Threedifferent configurations of varying porosity (0%, 50% and 80% porosity)were examined. Small disks (5 mm diameter) were punched from compressionmolded P4HB films of uniform thickness. Porous samples of P4HB wereproduced using the salt leaching technique described below. Thedegradation behavior in vitro was studied by incubating the disks in asterile, phosphate buffer (8 mM sodium phosphate, 2 mM potassiumphosphate, 140 mM NaCl, 10 mM KCl, pH 7.4, containing NaN₃ aspreservative) at 37° C. The degradation behavior in vivo was studied,after implantation in subcutaneous pockets in rats.

Preparation of Porous P4HB

Classified sodium chloride crystals (80–180 μm) were mixed with moltenP4HB. (Note that the polymer salt ratio can be adjusted to produce thedesired porosity, while particle size may be adjusted to produce poresof varying size.) The polymer salt mixture was pressed into a thin film.After allowing the material to solidify, the film was removed from themylar backing. The film was exhaustively extracted with water to removethe salt, leaving a porous film of P4HB.

Accelerated Degradation of P4HB

The degradation of P4HB was studied in vivo. Three differentconfigurations of varying porosity (0%, 50%, and 80% porosity) wereexamined. Small disks (5 mm diam.) were punched from compression moldedP4HB films of uniform thickness. Porous samples of P4HB were producedusing a salt leaching technique. The degradation behavior in vivo wasstudied after implantation in subcutaneous pockets in rats. Samples wereremoved at various times. The molecular mass was measured by GPC andmass loss was measured by quantification of the remaining 4HB by CGanalysis. The results are shown in FIG. 3. As shown in FIG. 3, thesample mass loss varied with porosity. Film, 50%, and 80% porous samplesshowed a 5%, 20%, and 75% mass loss, respectively, over the six weekperiod, while the average molecular mass loss of these samples alsodecreased significantly (20 to 50%). These data demonstrate that thedegradation rate of PHAs can be modified and controlled by alteringporosity and increasing surface area.

Results

The P4HB implants showed a very minimal inflammatory response, much lessso than for a PGA non-woven mesh. This is a very good indication of thebiocompatibility of these materials. Samples were removed at varioustimes and evaluated histologically both as to the implants andsurrounding tissue. The molecular mass was measured by GPC and mass losswas measured by quantification of the remaining 4HB by GC analysis. Theresults are shown in Tables 6 and 7. As shown in Table 6, P4HB does notdegrade significantly over a ten week period in vitro. All of thesamples maintained their starting weight, and there was about a 20 to40% decrease in average molecular mass. The samples incubated in vivoshowed much more pronounced degradation. The mass loss varied withporosity. Film, 50%, and 80% porous samples showed a 20%, 50%, and 100%mass loss, respectively, over the ten week period, while the averagemolecular mass loss of these samples also decreased significantly (20 to50%).

Light microscopic and environmental scanning electron microscopy (ESEM)examination of the samples show almost no discernible change for the invitro samples over the ten week incubation period. In contrast, the invivo implants show distinct signs of degradation. The surface of thesematerials became progressively degraded during the ten week implantationperiod. After one week, the film samples showed some signs of crackingand crazing, which progressed to surface erosion and pitting over thefollowing nine weeks.

The in vitro degradation data suggest that P4HB is fairly stable tosimple hydrolysis, unlike other polyesters used in bioresorbableapplications, such as PGA, PLA and their copolymers. However, thedegradation of the implants indicated that P4HB can be degraded in vivo,suggesting a biologically mediated mode of degradation. The data showsincreasing degradation with increasing porosity, which indicates thatsurface area of the polymer implant plays a role in its degradation invivo. This suggests that the degradation of P4HB polymers in vivo occursat the surface of the implant, unlike PGA or PLA materials which degradethroughout the implant by hydrolysis, with associated molecular massdecrease and loss of mechanical properties. These data suggest that thedegradation rate of P4HB can be modified and controlled by altering itssurface area. Also, it is expected that this type of surface degradationwill result in a relatively slow rate of molecular mass loss allowingfor the maintenance of polymer material properties longer than existingabsorbable, medical polyesters. The P4HB implants were very welltolerated and showed only a very minimal foreign body reaction. Thesefindings show that these materials have significant advantages overexisting biomedical polyesters.

TABLE 6 Degradation of P4HB In Vitro: Percent Original Mass Remainingand Molecular Mass Film 50% Por. 50% Por. 80% Por. 80% Por. ImplantationFilm Wt % Molec. Wt % Molec. Wt % Molec. (weeks) Remain.^(a) Mass^(b)Remain.^(a) Mass^(b) Remain.^(a) Mass^(b) 0 108 1144592 96  963145 1231291117 1  97 1160707 93 1103860  99  968245 2 101 1008496 98 1055614106 1072328 4 100  887005 96  725089 116  987665 6 109  896521 97 764260  95 1049079 10   92  772485 90  605608 100  727543^(a)Determined by GPC analysis. See Table 3 for details. ^(b)Determinedby quantitative GC analysis. See Table 2 for details.

TABLE 7 Degradation of P4HB In Vivo: Percent Original Mass Remaining andMolecular Mass Film 50% Por. 50% Por. 80% Por. 80% Por. ImplantationFilm Wt % Molec. Wt % Molec. Wt % Molec. (weeks) Remain.^(a) Mass^(b)Remain.^(a) Mass^(b) Remain.^(a) Mass^(b) 0 108  1144592 96 963145 123 1291117  1 103  1091107 109  1026821  88 1132492  2 95 1054873 94 97383035 943960 4 92 1007736 73 989629 39 881919 6 90  797170 74 901330 28689157 10  80  716296 48 647175  0 nd ^(a)Determined by GPC analysis.See Table 3 for details. ^(b)Determined by GC analysis. See Table 2 fordetails. Explants often weighed more than the original implant due tothe presence of adherent tissue or coagulated blood. Therefore, the massof P4HB in the explant was determined by quantitative GC analysis.Weight percent remaining P4HB was taken as this mass divided by originalimplant.

EXAMPLE 5 Compression Molding

P4HB was pressed into a thin film using Carver hydraulic press. Theplatens were heated to 115° C. P4HB was pressed between two sheets ofmylar using metal spacers. Spacer thickness and pressure of the presswere adjusted to control film thickness. The film was removed from thepress and allowed to cool at room temperature. After solidifying (withina matter of seconds), the film was easily peeled from the mylar backingmaterial. Mechanical data for this material is shown in Table 1. Therapid solidification of P4HB demonstrates its rapid crystallization.

TABLE 1 Thermal and Mechanical Properties of Selected Medical PolymersTensile Modulus Elongation Polymer Tm (° C.) Tg (° C.) Str. (psi) (psi)(%) Degradation ¹P4HB 60 −51 7,500 9,400 1000 depends on config.¹pP4HB50^(a) 60 −51 895 2155 164 depends on config. ¹pP4HB80^(b) 60 −51180 257 100 depends on config. ⁶P4HB-3HB 50 −42 9,000 14,500 1080 Notreported 10% ¹PHB 175 0 4,000 110,000 4 >52 wks ²PGA 230 35 10,0001,000,000 17  8 wks ³PDLLA Am 53 5,000 300,000 5  <8 wks ³PLLA 175 5510,000 500,000 8  >8 wks ²DLPLG 50/50 Am 48 7,000 300,000 5 3–8 wks⁵LDPE 2,000 400–700  Nondegradable ⁵HDPE 4,000 100–1000 Nondegradable⁵UHMWPE 7,250 450 Nondegradable PP 4,000 20,000 200–700  NondegradablePET 8,500 50 Nondegradable PTFE 3,000 @ 50,000 300 Nondegradable Yield^(a)pP4HB50, 50% porous P4HB, see example 7. ^(b)pP4HB80, 80% porousP4HB, see example 7. Table References: ¹From this work measuredaccording to ASTMD638 at ambient temperature and a strain rate of 0.05or 0.1 in./min.. ²Hutmacher et al. Int. J. Oral Max. Imp. 1996, 11:667–78. ³Nobes et al. submitted. ⁴Mark, Physical Properties of PolymersHandbook, American Inst. of Physics, Woodbury, New York, 1996.⁵Schwartz, S. S. and Goodman, S. H. Plastic Materials and Processes, VanNostrand Reinhold Company, New York, 1982. ⁶Saito, Y. and Doi, Y. Int.J. Biol. Macromol. (1994) 16: 99–104.

EXAMPLE 6 Compression Molding of Porous P4HB

Classified sodium chloride crystals (80–180 μm) were mixed with moltenP4HB as described in Examples 4 and 5. (The polymer salt ratio can beadjusted to produce the desired porosity, while particle size may beadjusted to produce pores of varying size.) The polymer salt mixture waspressed into a thin film using the conditions described in Example 6.After allowing the material to solidify, the film was removed from themylar backing. The film was exhaustively extracted with water to removethe salt, leaving a porous film of P4HB. Salt removal was monitored byanalysis of chloride in the supernatant and confirmed by elementalanalysis of the leached film (less than 0.5% chloride). Mechanical datafor 50% and 80% porous P4HB (pP4HB50 and pP4HB80, respectively) is shownin Table 1.

EXAMPLE 7 Cell Seeding of P4HB Scaffolds

Porous P4HB, as described in Example 6, was sterilized by cold ethyleneoxide treatment. It was seeded with ovine vascular cells and cultured invitro. Preliminary data indicated very good attachment of these cells tothe material. This is a further demonstration of the biocompatibility ofthis material. The number of cells attached to the material can bequantified using an assay for DNA and compared with the standard fortissue engineering scaffolds, PGA mesh.

EXAMPLE 8 P4HB Fiber Orientation

Compression molded strips of P4HB were uniaxially stretched. The samplenarrowed and became clear, showing signs of necking. After thisstretching process, the polymer appeared stronger and somewhat moreflexible, demonstrating uniaxial orientation of the sample.

EXAMPLE 9 Production of P4HB Foam

A thermal phase separation method was used to make P4HB foam. First,P4HB was dissolved in dioxane at 1 to 5% wt./vol. This polymer solutionwas cast as a thick film and solidified by cooling on ice below themelting point of dioxane. The solvent was evaporated from this solidmaterial at low pressure to yield a porous foam with the approximatedimensions of the starting thick film. ESEM analysis of this materialshowed a highly porous, sponge-like structure. The polymer concentrationand cooling process can be varied to alter the porosity of the foam.Prior to freezing, the polymer solution can be shaped into a variety offorms, broken up into particulate material or used as a coating.Therefore, this thermal phase separation technique can be used toproduce a great variety of highly porous, 3-dimensional shapes of P4HB.

EXAMPLE 10 P4HB Coating of a PGA Non-Woven Mesh

P4HB was dissolved in tetrahydrofuran at 1% wt/vol. A 1 mm thicknon-woven mesh of PGA (Albany International, bulk density 52 mg/cc) wasdipped into this solution so that the air voids were eliminated. Thecoated mesh was allowed to air dry, and the coating procedure wasrepeated. Light microscopic and ESEM analyses of the coated mesh showedthat during the drying process the polymer migrated to the fiberintersections, and functioned to bind the fibers together. This fiberbonding technique was found to dramatically improve the strength andhandleability of the PGA mesh. Tensile testing according to ASTM D638,showed that the tensile strength, Young's modulus, and ultimateelongation of this material were 130 psi, 240 psi, and 171%,respectively. This was a dramatic improvement over the uncoated materialwhich was too fragile to test these parameters.

EXAMPLE 11 P4HB Foam Coating of a PGA Non-Woven Mesh

P4HB was dissolved in dioxane at 2.5% wt/vol. A 1 mm thick non-wovenmesh of PGA (Albany International, bulk density 52 mg/cc) was dippedinto this solution so that the air voids were eliminated. The coatedmesh was cooled on ice so that the coating solution solidified. The meshwas freeze-dried to remove the dioxane. Light microscopic analysis ofthe coated mesh showed that during the freeze-drying process the polymerformed a web-like foam throughout the PGA mesh. This foamed material hasgood handleability. The high surface area and improved mechanicalproperties are attractive for a variety of applications.

EXAMPLE 12 Formation of P4HB Microspheres

P4HB was dissolved in dichloromethane at 1% wt/vol. A 1 ml volume ofthis solution was mixed with 5 ml of a 0.5% wt/vol. solution of sodiumdodecylsufate (SDS). The two phase mixture was mechanically mixed toyield an emulsion. A stream of nitrogen was bubbled through the mixturefor 1 hour with rapid stirring to facilitate removal of thedichloromethane. The mixture was stirred open to the air overnight toallow for the complete removal of dichloromethane. The resultantsuspension contained P4HB microspheres of about 1–10 μm, as determinedunder a phase contrast optical microscope.

Conclusions from Examples

Polyhydroxyalkanoates such as the homopolymer P4HB and copolymerscontaining 4HB have physical properties and degradation characteristicswhich make them very attractive as implants for use in medicalapplications. These polymers can be fabricated into fibers, sheets,foams, coating, structures, filaments and the like for use of these asimplantable medical materials.

Modifications and variations of the present invention will be obvious tothose of skill in the art from the foregoing detailed description. Suchmodifications and variations are intended to come within the scope ofthe following claims.

1. A device comprising a biodegradable polyhydroxyalkanoate polymercomposition that has a controlled degradation rate of less than oneyear, under physiological conditions, wherein the degradation rate ofthe composition is manipulated through addition of components to thepolymeric composition, selection of the chemical composition, molecularweight, processing conditions, or form of the composition, wherein thedevice is selected from the group consisting of rotator cuff repairdevices, temporary wound support devices, bladder patches, pledgets,soft tissue reinforcement devices, vascular patches, devices for artrialwall repair, bone marrow scaffolds, ligament repair devices, rods,washers, screws, pins, struts, plates, and staples used in spinal fusioncages, stents, sewing rings, stiffners used in heart valve supports,cell encapsulation devices, coated devices, defect filling devices,organ patches, organ salvage devices, staple line reinforcement devices,pelvic floor reconstruction devices, devices for closure of ventricularseptal defects, drug delivery devices, devices for delivery ofbiological factors, and devices comprising encapsulated proteins,antibodies, enzymes, peptides, polysaccharides, saccharides, organicdrugs, inorganic drugs, nucleic acids, antigens, inhibitors, clotdissolving agents, hormones, nucleic acid, and/or lipids.
 2. A methodfor making a device that has a controlled degradation rate of less thanone year under physiological conditions, comprising providing abiocompatible polyhydroxyalkanoate composition, wherein the degradationrate of the composition is manipulated through addition of components tothe polymeric composition, selection of the chemical composition,molecular weight, processing conditions, or form of the composition, andforming or incorporating the polyhydroxyalkanoate composition into adevice selected from the group consisting of vein valves, rotator cuffrepair devices, temporary wound support devices, bladder patches,pledgets, soft tissue reinforcement devices, vascular patches, devicesfor artrial wall repair, bone marrow scaffolds, meniscus regenerationdevices, ligament repair devices, rods, washers, screws, pins, struts,plates, and staples used in spinal fusion cages, stents, sewing rings,stiffners used in heart valve supports, cell encapsulation devices,coated devices, defect filling devices, organ patches, organ salvagedevices, staple line reinforcement devices, pelvic floor reconstructiondevices, devices for closure of ventricular septal defects, drugdelivery devices, devices for delivery of biological factors, anddevices comprising encapsulated proteins, antibodies, enzymes, peptides,polysaccharides, saccharides, organic and inorganic drugs, nucleicacids, antigens, inhibitors, clot dissolving agents, hormones, nucleicacid, and/or lipids.
 3. The method of claim 2 wherein the processingforming or incorporating process is selected from the group consistingof solvent casting, melt processing, fiber processing, fiber spinning,fiber weaving, extrusion, injection molding, compression molding,lamination, and microparticle formation.
 4. The method of claim 2further comprising incorporating an active agent into thepolyhydroxyalkanoate.
 5. The device of claim 1 wherein the chemicalcomposition of the polyhydroxyalkanoate is altered through selection ofmonomers which are incorporated into the polymer, by alteration of thelinkages, chemical backbone or pendant groups.
 6. The device of claim 1wherein the polyhydroxyalkanoate composition comprises additivesaltering the degradation rate of the composition, wherein the additivesare selected from the group consisting of hydrophilic substances,hydrophobic substances, and coatings which alter water uptake by thecomposition.
 7. The device of claim 1 wherein the polyhydroxyalkanoatecomprises a polymer selected from the group of consisting ofpoly-4-hydroxybutyrate, poly-4-hydroxybutyrate-co-3-hydroxybutyrate,poly-4-hydroxybutyrate-co-2-hydroxybutyrate, and copolymers and blendsthereof.
 8. The device of claim 1 wherein the polyhydroxyalkanoatecomprises a polymer selected from the group consisting ofpoly-3-hydroxybutyrate-co-3-hydroxyhexanoate,poly-3-hydroxybutyrate-co-3-hydroxyoctanoate,poly-3-hydroxybutryrate-co-3-hydroxydecanoate, and copolymers and blendsthereof.
 9. The device of claim 1 wherein the polyhydroxyalkanoatecomprises one or more units which alter the chemical stability of thepolymer backbone.
 10. The device of claim 1 comprising units promotingchain scission.
 11. The device of claim 10 wherein the units containmore than two functional groups.
 12. The device of claim 1 wherein aheteroatom is incorporated into the polymer backbone chain.
 13. Thedevice of claim 12 wherein the heteroatom is selected from the groupconsisting of oxygen, sulfur or nitrogen.
 14. The device of claim 10wherein the units are incorporated into the polymer backbone withchemical linkages selected from the group consisting of ester, amide,ether, carbamate, anhydride, and carbonate.
 15. The device of claim 10wherein the units are selected from the group consisting of2-hydroxyacids, 2-hydroxyalkaoxyacetic acids, amino acids, aminoalcohols, diacids, triols, and tetraols.
 16. The device of claim 15wherein the 2-hydroxyacids are 2-hydroxyalkanoic acids.
 17. The deviceof claim 16 wherein the 2-hydroxyalkanoic acid is lactic acid orglycolic acid.
 18. The device of claim 15 wherein the2-hydroxyalkoxyacetic acids are selected from the group consisting of2-hydroxyethoxy acetic acid and 3-hydroxypropoxy acetic acid.
 19. Thedevice of claim 1 wherein the polymer comprises pendant groups thatcatalyze the degradation of the polymer backbone.
 20. The device ofclaim 19 wherein the pendant groups are selected from acidic and basicgroups.
 21. The device of claim 19 comprising reactant pendant groupsthat cause polymer chain scission.
 22. The device of claim 21 whereinthe reactant pendant groups are selected from nucleophiles andelectrophiles.
 23. The device of claim 19 wherein the pendant groups areselected from the group consisting of alcohols, acids, and amine groups.24. The device of claim 1 comprising providing additives altering thechemical stability of the polyhydroxyalkanoate.
 25. The device of claim24 wherein the additives promote chain scission.
 26. The device of claim24 wherein the additives are selected from the group consisting ofacids, bases, electrophiles, nucleophiles, plasticizers, polymers, poreforming agents, and agents designed to reduce the polymer crystallinity.27. The device of claim 1 comprising providing pore forming agents. 28.The device of claim 27 wherein the pore forming agents are lyophilizableparticles.
 29. The device of claim 27 wherein the pore forming agentsabsorb water.
 30. The device of claim 1 further comprising one or moreactive agents.
 31. The device of claim 30 wherein the active agent isselected from the group consisting of growth factors, alginates, silversalts, antiseptics, analgesics, and preservatives.